# Patent

## Title
Synchronizing multiple threads efficiently

## Abstract

In one embodiment, the present invention includes a method of assigning a location within a shared variable for each of multiple threads and writing a value to a corresponding location to indicate that the corresponding thread has reached a barrier. In such manner, when all the threads have reached the barrier, synchronization is established. In some embodiments, the shared variable may be stored in a cache accessible by the multiple threads. Other embodiments are described and claimed.

## Background
BACKGROUND
The present invention relates to computer systems, and more particularly to such systems executing multiple threads.
Computer systems including multiprocessor (MP) and single processor systems may include a plurality of “threads,” each of which executes program instructions independently from other threads. Use of multiple processors allows various tasks or functions, and even multiple applications, to be handled more efficiently and with greater speed. Utilizing multiple threads or processors means that two or more processors or threads can share the same data stored within the system. However, care must be taken to maintain memory ordering when sharing data.
For data consistency purposes, if multiple threads or processors desire to read, modify, and write to a single memory location, the multiple agents should not be allowed to perform operations on the data simultaneously. Further complicating the use of multiple processors is that data is often is stored in a cache associated with a processor. Because such caches are typically localized to a specific processor, multiple caches in a multiprocessor computer system can contain different copies of a given data item. Any agent accessing this data should receive a valid or updated (i.e., latest) data value, and data being written from the cache back into memory must be the current data so that cache coherency is maintained.
Memory instruction processing acts in accordance with a target instruction set architecture (ISA) memory order model. For reference, Intel Corporation's two main ISAs: Intel® architecture (IA-32 or x86) and Intel's ITANIUM® processor family (IPF) have very different memory order models. In IA-32, load (i.e., read) and store (i.e., write) operations must be visible in program order, while in the IPF architecture, they do not in general. Further, while executing multiple threads in a chip multiprocessor (CMP) or other MP system, ordered memory instructions are used in synchronization and communication between different threads.
Multithreaded (MT) software uses different mechanisms to interact and coordinate between different threads. Two common forms of MP synchronization are barriers and semaphore spin-locks. A barrier mechanism helps a program synchronize different threads at predefined points in the program. Typically, each thread either increments or decrements a memory variable in an atomic fashion when it reaches such a point. Every thread then waits for the memory variable to reach a predetermined barrier level. Synchronization is achieved once all threads have completed the updates. When the barrier is reached, all threads can then proceed.
A semaphore spin-lock mechanism is used to guarantee mutual exclusion across multiple threads while accessing a shared memory variable or structure (i.e., a shared element). In order to provide a unique and consistent view of the shared element, it is guarded by a lock variable. Every thread needing access to the shared element must acquire the guarding lock (i.e., locking) via an atomic semaphore operation. When a lock is acquired, the remaining threads can only acquire the lock after it is released (i.e., unlocking) by the original requester. Only the thread that acquired the lock performs operations/updates on the shared element (software convention), thus mutual exclusion is ensured. Locking is performed by designating a particular value to represent a locked state, and a different value to represent an unlocked state. Each thread seeking to access the shared element acquires the lock by updating the lock variable atomically to the lock value (after possibly checking that the lock has not already been acquired).
Most ISA's provide specific semaphore instructions to achieve MP synchronization between multiple threads or processors. Among these, an atomic-add is a popular instruction for a barrier synchronization mechanism. However, known barrier synchronization methods and semaphore spin-locks cause inefficiencies. Barrier mechanisms typically require significant traffic, such as inter-processor cache traffic, as the lock variable moves to different cores of the multiprocessor. Similarly, spin-lock mechanisms require significant traffic between different processor cores. Still further, an atomic-add instruction requires that the shared variable be brought deep into processor cores to perform the add operation, again requiring significant traffic, as well as utilizing processor resources. Accordingly, a need exists for improved manners of synchronization between multiple threads.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a flow diagram of a method in accordance with one embodiment of the present invention.
FIG. 2 is a flow diagram of a synchronization method in accordance with one embodiment of the present invention
FIG. 3 is a flow diagram of a locking method in accordance with one embodiment of the present invention.
FIG. 4 is a block diagram of a representative system in accordance with one embodiment of the invention.
FIG. 5 is a block diagram of a multiprocessor system in accordance with another embodiment of the invention.



DETAILED DESCRIPTION
In various embodiments, a shared memory variable (“shared variable”) may be used to maintain synchronization between multiple threads. The shared variable may have multiple portions, with each portion being independently associated with one of the threads. In such manner, the shared variable may represent the synchronization status of each of the threads independently.
In some embodiments, the shared variable may be updated using a set mechanism, rather than an arithmetic operation, such as an increment or decrement operation. In such manner, the status of various threads may be updated without causing the shared variable to be cached or otherwise brought into a processor core. That is, set/reset operations may be performed in a remote or last level cache. Furthermore, these operations may be performed without expensive adders or other circuitry that would need to be located near an execution core.
In one embodiment, an instruction referred to herein as a fetchset instruction may be a low-cost instruction to improve MT synchronization, particularly in cases of low thread count. The fetchset instruction as executed by a given thread performs a read on a shared variable and sets or stores a predetermined value into a portion of the shared variable corresponding to the thread. For example, in an embodiment in which eight or fewer threads are executing, the shared variable may be eight bytes. Each byte of the shared variable may correspond to one of the threads. In one embodiment, the predetermined value to be set may be all “ones”. For example, fetchset1 stores FFhex (i.e., 11111111) at the specified memory address of a given portion. Such an instruction may be low cost, since setting (i.e., overwriting) memory state is less expensive than performing read-add-update operations.
While discussed above as devoting a byte per thread, in other embodiments a different arrangement may exist. For example in one embodiment, a single bit may be used to represent a thread. Further, while described above as setting the predetermined value to ones, in other embodiments any other desired value may be used.
Referring now to FIG. 1, shown is a flow diagram of a method in accordance with one embodiment of the present invention. More specifically, FIG. 1 shows a method 10 for performing a fetchset instruction in accordance with one embodiment. Such a fetchset instruction may be used to access a shared memory variable and store a value in a portion of the variable corresponding to a given thread. As shown in FIG. 1, a fetchset operation may be initiated (oval 20). Then a first memory variable may be read (block 30). The first memory variable may correspond to a shared variable used to indicate synchronization status for multiple threads at a barrier. A thread may read the variable to determine whether all portions of the variable equal a given value, or to determine whether a particular portion of the variable is in a locked state, as will be discussed further below.
Still referring to FIG. 1, next a portion of the first memory variable corresponding to a current thread may be set with the predetermined value (block 40). For example, a first portion of the memory variable (e.g., a first byte) may correspond to a first thread of a program. Upon executing a fetchset instruction, the first thread may set that portion. In one embodiment, the portion may be set with all ones. Method 10 then ends (oval 50).
While not shown in FIG. 1, it is to be understood that upon arriving at the barrier, each thread performs a fetchset instruction on the variable, and the predetermined value is stored at the specified slot within the variable. Synchronization is established when all bytes of the memory variable are set.
Referring now to FIG. 2, shown is a flow diagram of a synchronization method in accordance with one embodiment of the present invention. As shown in FIG. 2, method 100 may be used to synchronize multiple threads of a program. Method 100 begins (oval 105) by executing instructions of a given thread (block 110). Next it may be determined whether the thread reaches a barrier (diamond 120). For example, a barrier may correspond to a predetermined portion of the program at which synchronization is desired. If no such barrier is reached, the thread continues execution of instructions by looping back to block 110.
Alternately, if the thread does reach a barrier, control passes to block 130 where a fetchset operation is performed on a first memory variable. The first memory variable may be stored in a cache accessible by the multiple threads. Such a fetchset operation may include the steps described above with respect to FIG. 1. That is, the fetchset operation may cause the thread to read the contents of the first memory variable and set a value to a portion of the first memory variable corresponding to the current thread. Next, it may be determined whether the current thread is the last to reach the barrier (diamond 140). For example, in one embodiment the thread may read the variable contents and determine whether all portions equal a set value. However, in some embodiments the set value may be present in all portions of the first variable except that portion corresponding to the current thread. If the current thread is not the last to reach the barrier, control passes to block 150, where the thread spins on a second memory variable until it is updated. The second memory variable may also be stored in the cache.
If the current thread is the last to reach the barrier, the second memory variable may be updated (block 160). For example, the second memory variable may have its contents updated from zero to one. Control then passes to block 170, where the synchronization is completed, as all threads of the program have reached the barrier, and the contents of the first and second memory variables are set. Accordingly, method 100 concludes (oval 180).
Because of the low cost of implementing a fetchset instruction and as it may be defined on byte granularity, setting of the variable may be exported to a shared last level cache (LLC) of a CMP or other processor. In such manner, a cache line including the variable can be prevented from being copied into a core (near the execution pipeline) on every barrier update. Thus, inter-processor cache traffic in moving the lock variable across different cores may be avoided. Software may be used to ensure that the cache line remains pinned in the LLC by avoiding false sharing on the cache block and avoiding any other reads or updates to the memory variable (that would source a copy of the cache line into one of the core caches).
Other embodiments may be used to provide fine-grain locking using a single variable. For example, a 16-byte fetchset instruction may represent 16 fine-grain locks. In such an embodiment, a data structure containing 16 data elements can be locked using one shared variable. Each element of the data structure is allocated a byte within the lock variable. Then, only that portion of the structure may be locked using a fetchset instruction. In such manner, non-blocking access to the same structure is permitted, as long as the particular element to be accessed is not locked. If the original value returned by a fetchset operation to a first thread is clear in the particular byte location, then the first thread may successfully lock that element. If however the byte position was already set, the entry was previously locked by a different thread. Accordingly, the locking operation would need to be retried later by the first thread.
Referring now to FIG. 3, shown is a flow diagram of a locking method in accordance with one embodiment of the present invention. As shown in FIG. 3, method 200 may be used to lock a data structure using a plurality of fine-grain locks. Method 200 begins (oval 205) by an instruction that seeks access to a data structure that is subject to a lock (block 210). A fetchset operation, which may be similar to that shown in FIG. 1, may be performed on a shared variable associated with the data structure (block 220). However, for the embodiment of FIG. 3, block 40 of FIG. 1 may instead be used to set a predetermined value to a portion of the memory variable corresponding to a given data element of the data structure.
After reading the shared variable, it may be determined whether the portion of the variable corresponding to the data element is locked (diamond 230). For example, in one embodiment it may be determined whether that portion is set (i.e., has a value of ones). If the portion indicates that the data element is locked, the thread is not allowed to access the data element, and may accordingly retry the fetchset operation later (block 240). In such manner, if a different thread has concluded an operation on the data element, the next time the thread accesses the shared variable, the specific portion may be in a reset state.
If instead at diamond 230 it is determined that the portion of the shared variable is not locked, that portion may be locked (block 250). Then the current thread may update the data element that corresponds to that portion of the data structure (block 260). For example, the thread may update the data element and write the updated value back to its memory location. Then, the thread may unlock that portion of the shared variable (block 270) so that other threads may obtain a lock on the given data element. Accordingly, method 200 ends (oval 280).
In various embodiments, because a fetchset or other such instruction supports a predefined update payload (i.e., the predefined value), the payload may be synthesized at the particular cache executing the instruction. As a result, the update to the shared variable is protected against payload corruption through soft errors as the instruction flows through the machine.
Furthermore, because the synchronization status of each thread is isolated, it provides a robust barrier synchronization mechanism. Any thread that fails to report to the barrier does not have its byte location set in the shared variable. In such manner, any errant thread resulting in a software failure (either due to software or hardware error) may be isolated and identified based on the status of the shared variable. Software may then selectively perform recovery on the particular thread.
A 16 byte variant of the fetchset instruction (i.e., a fetchset.double) may be defined to operate on a 16 byte granularity, and thus provide synchronization among 16 threads.
In some embodiments, a hierarchical barrier mechanism may be used to establish synchronization in software with large thread counts. In such embodiments, a fetchset-based mechanism may serve as an effective barrier mechanism at a lower level of the hierarchy to establish synchronization within a CMP processor or within a local cluster of a clustered system.
Referring now to Table 1 below, shown is an instruction syntax for a fetchset instruction in accordance with an embodiment of the present invention. As shown in Table 1, the instruction syntax is within the scope of the IPF ISA.









TABLE 1









fetchset.dbl.pl.sz.sem r1, [ar.csd] = [r3]










  dbl:
s - 8byte read




d - 16byte read. Second 8 byte target ar.csd.



  pl:
0 - clear (‘0 payload)




1 - set (‘1111 .... payload)



  sz:
0 - 1 byte




1 - 2 byte




2 - 4 byte




3 - 8 byte




4 - 16 byte (only valid with dbl set to d).



  sem:
acq - acquire




rel - release.







Note:



ar.csd only applies when dbl is set to d)






As shown in Table 1, the instruction syntax indicates: whether an 8 byte or 16 byte read operation is to be performed; the payload to be written into the specified location; the size; and the type of semaphore, for example, whether it is an acquire or release operation.
Referring now to Table 2 below, shown is a pseudocode example of establishing a barrier synchronization using a fetchset instruction in accordance with an embodiment of the present invention. Again, this pseudocode is in the IPF context, although the scope of the present invention is not so limited.







TABLE 2







//r20 contains the barrier address.


//r40 contains the shared memory variable.









stf.spill [r20] = f0



stf.spill [r40] = f0



//note: these lines write a 16-byte zero



value into the target registers



mov r30 = ‘hFFFFFFFFFFFFFFFF;;



mov ar.csd = r30;;



//note: this line moves the second operand









for the fetchset operation into the target register







// Start independent thread-level execution.









.



.



.







// Complete thread execution









fetchset.d.1.rel r10, ar.csd = [r20], 0;;



cmp.eq p1, p2 = r10, r30



//note: if the register values are equal, p1 is set to









one and p2 is set to zero, otherwise vice versa









mov r35 = ar.csd;;



cmp.eq.and p1, p2 = r10, r35;;



p1: stf.spill [r40] = f1;;



p1: br sync_cmp



//note: if p1 equals one, operation branches to









sync_cmp (i.e., synchronization is complete)








spin_loop:
ldf f19 = [r40]



fcmp.eq p1, p2 = f19, f1;



//note: if target values are equal, p1 equals









zero and p2 equals one and operation branches



to sync_cmp (i.e., synchronization



is complete), otherwise vice versa and



spin_loop is repeated









p2: br spin_loop


sync_cmp:
//synchronization completed.









Generally, the code of Table 2 performs the following activities. First, the code initializes the location in the cache where the shared variable is to be stored and further initializes the shared variable to zero. Then, independent threads are executed. Upon completion of a thread, a fetchset instruction is performed by each thread to read the value of the shared variable and compare it to a predetermined value. If the shared variable equals that value, it means that the current thread is the last thread, and synchronization is completed. If not, the current thread updates its portion of the shared variable, and enters a spin loop until the last thread completes its execution to thus complete synchronization.
Referring now to FIG. 4, shown is a block diagram of a representative computer system 300 in accordance with one embodiment of the invention. As shown in FIG. 4, computer system 300 includes a processor 310 which may be a chip multiprocessor (CMP). As shown in FIG. 4, processor 310 may include a first core 310a (core A) and a second core 310b (core B). Processor 310 may be coupled over a memory system interconnect 320 to a cache coherent shared memory subsystem (“coherent memory”) 330 in one embodiment. In one embodiment, coherent memory 330 may include a dynamic random access memory (DRAM) and may further include coherent memory controller logic to share coherent memory 330 between multiple processors.
It is to be understood that in other embodiments additional processors may be coupled to coherent memory 330. Furthermore in certain embodiments, coherent memory 330 may be implemented in parts and spread out such that a subset of processors within system 300 communicate to some portions of coherent memory 330 and other processors communicate to other portions of coherent memory 330.
As shown in FIG. 4, core 310a may include a cache 314a and execution units 316a in accordance with an embodiment of the present invention. Cache 314a may be a low level cache (e.g., a level one (L1)) associated with core 310a. Of course, additional components may reside in core 310a, such as additional caches. More so, a last level cache (LLC) 305, which may be a level two (L2) cache, may be coupled to both cores 310a and 310b. LLC 305 may store a lock variable in accordance with an embodiment of the present invention. Furthermore, the value to be set into portions of the lock variable may be synthesized in LLC 305. As further shown in FIG. 4, similar processor components may be present in core 310b, which may be a second processor core of a multiprocessor system such as a chip multiprocessor (CMP).
Coherent memory 330 may also be coupled (via a hub link) to an input/output (I/O) hub 335 that is coupled to an I/O expansion bus 355 and a peripheral bus 350. In various embodiments, I/O expansion bus 355 may be coupled to various I/O devices such as a keyboard and mouse, among other devices. Peripheral bus 350 may be coupled to various components such as peripheral device 370 which may be a memory device such as a flash memory, add-in card, or the like. Although the description makes reference to specific components of system 300, numerous modifications of the illustrated embodiments may be possible.
In some embodiments, a multiprocessor system may be a point-to-point bus system, such as in a common system interface (CSI) system. Referring now to FIG. 5, shown is a block diagram of a multiprocessor system in accordance with another embodiment of the present invention. As shown in FIG. 5, the multiprocessor system is a point-to-point bus system, and includes a first processor 470 and a second processor 480 coupled via a point-to-point interconnect 450. First processor 470 may include multiple processor cores 474a and 474b (although more such cores may be present), a memory controller hub (MCH) 472, a cache memory 473, and point-to-point (P-P) interfaces 476 and 478. Similarly, second processor 480 includes the same components, namely processor cores 484a and 484b, a MCH 482, a cache memory 483, and P-P interfaces 486 and 488. Caches 473 and 483 may store lock variables in accordance with an embodiment of the present invention. Furthermore, values to be set into portions of the lock variables may be synthesized in caches 473 and 483.
As shown in FIG. 5, MCH's 472 and 482 couple the processors to respective memories, namely a memory 432 and a memory 444, which may be portions of main memory locally attached to the respective processors.
First processor 470 and second processor 480 may be coupled to a chipset 490 via P-P interfaces 452 and 454, respectively. As shown in FIG. 5, chipset 490 includes P-P interfaces 494 and 498. Furthermore, chipset 490 includes an interface 492 to couple chipset 490 with a high performance graphics engine 438. In one embodiment, an Advanced Graphics Port (AGP) bus 439 may be used to couple graphics engine 438 to chipset 490. AGP bus 439 may conform to the Accelerated Graphics Port Interface Specification, Revision 2.0, published May 4, 1998, by Intel Corporation, Santa Clara, Calif. Alternately, a point-to-point interconnect 439 may couple these components.
In turn, chipset 490 may be coupled to a first bus 416 via an interface 496. In one embodiment, first bus 416 may be a Peripheral Component Interconnect (PCI) bus, as defined by the PCI Local Bus Specification, Production Version, Revision 2.1, dated June 1995 or a bus such as the PCI Express bus or another third generation I/O interconnect bus, although the scope of the present invention is not so limited.
As shown in FIG. 5, various input/output (I/O) devices 414 may be coupled to first bus 416, along with a bus bridge 418 which couples first bus 416 to a second bus 420. In one embodiment, second bus 420 may be a low pin count (LPC) bus. Various devices may be coupled to second bus 420 including, for example, a keyboard/mouse 422, communication devices 426 and a data storage unit 428 which may include, in one embodiment code 430. Further, an audio I/O 424 may be coupled to second bus 420.
Embodiments may be implemented in a computer program that may be stored on a storage medium having instructions to program a computer system to perform the embodiments. The storage medium may include, but is not limited to, any type of disk including floppy disks, optical disks, compact disk read-only memories (CD-ROMs), compact disk rewritables (CD-RWs), and magneto-optical disks, semiconductor devices such as read-only memories (ROMs), random access memories (RAMs) such as dynamic and static RAMs, erasable programmable read-only memories (EPROMs), electrically erasable programmable read-only memories (EEPROMs), flash memories, magnetic or optical cards, or any type of media suitable for storing electronic instructions. Other embodiments may be implemented as software modules executed by a programmable control device.
While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.

## Claims

1. A method comprising:
assigning one of a plurality of locations within a shared variable stored in a last level cache shared by and accessible to at least two processors for each of multiple threads, each of the plurality of locations within the shared variable readable by all of the multiple threads but only writable by one of the multiple threads; and
writing a value to a corresponding location within the shared variable assigned to a first thread to indicate that the first thread reached a barrier, the shared variable to indicate a synchronization status for the multiple threads at the barrier, the value synthesized in the last level cache without copying a cache line including the shared variable to either of the at least two processors.

2. The method of claim 1, wherein each of the plurality of locations comprises a byte of the shared variable.

3. The method of claim 1, further comprising reading the plurality of locations within the shared variable by one of the multiple threads when the one thread reaches the barrier.

4. The method of claim 3, further comprising updating a second memory variable if the one thread is the last of the multiple threads to reach the barrier.

5. The method of claim 1, further comprising writing the value within the corresponding location within the shared variable without copying a cache line including the shared variable to a core.

6. The method of claim 1, further comprising performing a single instruction by the first thread to read the shared variable and to write the value to the corresponding location of the shared variable.

7. The method of claim 6, wherein the single instruction comprises a set instruction performed without an increment or decrement operation.

8. An article comprising a machine-readable storage medium containing instructions that if executed by a machine enable the machine to perform a method comprising:
executing a plurality of threads of a program; and
storing a first value in a first portion of a shared variable assigned to a first thread of the plurality of threads when the first thread reaches a barrier of the program, the shared variable to indicate a synchronization status for the plurality of threads at the barrier and stored in a last level cache shared by at least two processors and having a portion assigned to each of the plurality of threads, wherein each of the portions is readable by the plurality of threads and only writable by an assigned one of the threads, the first value synthesized in the last level cache without copying a cache line including the first shared variable to either of the at least two processors.

9. The article of claim 8, wherein the method further comprises reading the shared variable by the first thread to determine if the first thread is the last of the plurality of threads to reach the barrier.

10. The article of claim 8, wherein the method further comprises isolating a failure to a given thread based on a status of the shared variable that indicates that a portion of the shared variable corresponding to the given thread is not of the first value, and responsive thereto performing recovery on the given thread.

11. The article of claim 8, wherein the method further comprises storing the first value in a second portion of the shared variable when a second thread reaches the barrier.

12. The article of claim 11, wherein the method further comprises updating a second variable if the plurality of threads reach the barrier.

13. The article of claim 12, wherein the method further comprises causing at least one of the plurality of threads to spin on the second variable until the second variable is updated.

14. An apparatus comprising:
a cache coupled to a first processor and a second processor to store a shared memory structure having a plurality of portions, wherein each of the plurality of portions corresponds to one of a plurality of threads of a program and is writable by the corresponding thread and readable by the plurality of threads and the shared memory structure is to represent a synchronization status for each of the plurality of threads at a barrier independently, wherein the cache is to synthesize a value in at least one of the plurality of portions when a corresponding one of the plurality of threads reaches the barrier without copying a cache line including the shared memory structure to either of the first and second processors.

15. The apparatus of claim 14, wherein the cache comprises a last level cache.

16. The apparatus of claim 14, wherein the first processor and the second processor comprise cores of a chip multiprocessor.

17. An apparatus comprising:
first processor core;
second processor core coupled to the first processor core; and
a cache coupled to the first processor core and the second processor core to store a first shared memory structure having a plurality of portions, wherein each of the plurality of portions corresponds to one of a plurality of threads of a program executed on the first processor core and the second processor core and the first shared memory structure is to represent a synchronization status for each of the plurality of threads at a barrier independently and the cache is to synthesize a value in at least one of the plurality of portions when a corresponding one of the plurality of threads reaches the barrier without copying a cache line including the first shared memory structure to either of the first and second processor cores.

18. The apparatus of claim 17, wherein the first processor core and the second processor core comprise a chip multiprocessor and wherein the cache comprises a last level cache.

19. The apparatus of claim 17, wherein the cache is to store a status variable to indicate the synchronization status for the plurality of threads.

20. The apparatus of claim 17, wherein the cache is to store a second shared memory structure having multiple locations, wherein each of the multiple locations corresponds to one of a plurality of data elements of a data structure.

21. The apparatus of claim 20, wherein a thread is prevented from access to one of the plurality of data elements if the corresponding location of the second shared memory structure is indicative of a locked state.