mempool: make mempool_cpuid() inline

In some runs callq to mempool_cpuid shows up in 'perf kvm top' profile.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
mempool: drop redundant assert

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
mempool: erase page range using an iterator rather than a reference

free_page_ranges is an intrusive set. erasing via a reference requires
iteration over reference equal_range under the hood, which means
traversing the tree to the leafs. Whereas erasing via an iterator
requires no such lookups so should be faster.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
lockfree: introduce unordered_queue_mpsc

It's like queue_mpsc with two improvements:

 * consumer and producer links are cache line aligned to avoid false
   sharing. I was tempted to apply this to queue_mpsc too but then
   discovered that this queue is embedded in a mutex, and doing so
   would greatly bloat mutex size, so I gave up on this idea.

 * The contract of pop() is relaxed to return items in no particular
   order so that we can avoid the cost of reversing the chain.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
lockfree: introduce unordered_queue_spsc

It is meant to provide both the speed of a ring buffer and
non-blocking properties of linked queues by combining the two. Unlike
for ring_spsc, push() is always guaranteed to succeed.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
tests: introduce latch.is_released()

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
tests: introduce latch.await_for()

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
tests: add test for free() throughput across CPUs

There is one allocating threads and two freeing threads. Each thread
is allocated on a different core. The test measures throughput of
objects freed by both threads.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
mempool: remove contention inside free_different_cpu()

Fixes #308.

When the per-cpu-pair ring fills up, the freeing thread is blocked and
enters a synchronous object hand-off. That synchronous hand-off is
cause of contention. Instead of having a bounded ring we can use an
unordered_queue_mpsc which links the freed objects in a chain. In this
implementation push() always succeeds and we don't need to block.

In a test which allocates 1K blocks on once CPU and having two threads
freeing them on two other CPUs, there is a ~40% improvement of free()
throughput.

I tested various implementaions, based on different queues. Statistics
of free/sec reported by misc-free-perf (one sample = one run):

current:

  avg =  8133055.09
  stdev = 118322.06
  samples = 5

ring_spsc<1M> (no blocking):

  avg = 10442665.98
  stdev = 476334.93
  samples = 5

unordered_queue_spsc:

  avg = 10258212.69
  stdev = 418194.22
  samples = 5

unordered_queue_mpsc:

  avg = 11701334.99
  stdev = 725299.97
  samples = 5

Testing showed that unordered_queue_mpsc() performs best in this case.

Dead objects are collected by per-CPU worker thread (same as
before). The thread is woken up once every 256 frees. That threshold
was chosen so that the behavior would more or less correspond to what
was before.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Merge branch 'mempool' of https://github.com/tgrabiec/osv

"The main improvement is in the last patch which removes contention
inside free_different_cpu() on sync._mtx. It improves my
micro-benchmark by ~30%."

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>