BuildBatchGroup -- memcpy outside of lock

Summary: When building batch group, don't actually build a new batch since it requires heavy-weight mem copy and malloc. Only store references to the batches and build the batch group without lock held.

Test Plan:
`make check`

I am also planning to run performance tests. The workload that will benefit from this change is readwhilewriting. I will post the results once I have them.

Reviewers: dhruba, haobo, kailiu

Reviewed By: haobo

CC: leveldb, xjin

Differential Revision: https://reviews.facebook.net/D15063
VersionEdit not to take NumLevels()

Summary:
I will submit a sequence of diffs that are preparing master branch for column families. There are a lot of implicit assumptions in the code that are making column family implementation hard. If I make the change only in column family branch, it will make merging back to master impossible.

Most of the diffs will be simple code refactorings, so I hope we can have fast turnaround time. Feel free to grab me in person to discuss any of them.

This diff removes number of level check from VersionEdit. It is used only when VersionEdit is read, not written, but has to be set when it is written. I believe it is a right thing to make VersionEdit dumb and check consistency on the caller side. This will also make it much easier to implement Column Families, since different column families can have different number of levels.

Test Plan: make check

Reviewers: dhruba, haobo, sdong, kailiu

Reviewed By: kailiu

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15159
Fix memtable construction in tests
Fix test
Wrong number of levels is Invalid argument now, not corruption
Fix CompactRange to apply filter to every key

Summary:
When doing CompactRange(), we should first flush the memtable and then calculate max_level_with_files. Also, we want to compact all the levels that have files, including level `max_level_with_files`.

This patch fixed the unit test.

Test Plan: Added a failing unit test and a fix, so it's not failing anymore.

Reviewers: dhruba, haobo, sdong

Reviewed By: haobo

CC: leveldb, xjin

Differential Revision: https://reviews.facebook.net/D14421
[RocksDB Performance Branch] DBImpl.NewInternalIterator() to reduce works inside mutex

Summary: To reduce mutex contention caused by DBImpl.NewInternalIterator(), in this function, move all the iteration creation works out of mutex, only leaving object ref and get.

Test Plan:
make all check
will run db_stress for a while too to make sure no problem.

Reviewers: haobo, dhruba, kailiu

Reviewed By: haobo

CC: igor, leveldb

Differential Revision: https://reviews.facebook.net/D14589

Conflicts:
	db/db_impl.cc
Fix the return type of WriteBatch::Data().

Summary: Quick fix for https://reviews.facebook.net/D15123

Test Plan: Make check

Reviewers: sdong, vkrest

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15165
Optimize MayContainHash()

Summary:
In latest leaf's, MayContainHash() consistently consumes 5%~7% CPU usage.

I checked the code and did an experiment with/without inlining this method.

In release mode, with `1024 * 1024 * 256` bits and `1024 * 512` entries, both call 2^30 MayContainHash() with distinctive parameters.

As the result showed, this patch reduced the running time from 9.127 sec to 7.891 sec.

Test Plan: make check

Reviewers: sdong, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15177
Decrease reliance on VersionSet::NumberLevels()

Summary:
With column families VersionSet will not have a constant number of levels (each CF can have different options), so we'll need to eliminate call to VersionSet::NumberLevels()

This diff decreases number of callsites, but we're not there yet. It associates number of levels with Version (each version is associated with single CF) instead of VersionSet.

I have also slightly changed how VersionSet keeps track of manifest size.

This diff also modifies constructor of Compaction such that it takes input_version and automatically Ref()s it. Before this was done outside of constructor.

In next diffs I will continue to decrease number of callsites of VersionSet::NumberLevels() and also references to current_

Test Plan: make check

Reviewers: haobo, dhruba, kailiu, sdong

Reviewed By: sdong

Differential Revision: https://reviews.facebook.net/D15171
Move functions from VersionSet to Version

Summary:
There were some functions in VersionSet that had no reason to be there instead of Version. Moving them to Version will make column families implementation easier.

The functions moved are:
* NumLevelBytes
* LevelSummary
* LevelFileSummary
* MaxNextLevelOverlappingBytes
* AddLiveFiles (previously AddLiveFilesCurrentVersion())
* NeedSlowdownForNumLevel0Files

The diff continues on (and depends on) D15171

Test Plan: make check

Reviewers: dhruba, haobo, kailiu, sdong, emayanke

Reviewed By: sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15183
Moving Compaction class to separate header file

Summary:
I'm sure we'll all agree that version_set.cc needs simplifying. This diff moves Compaction class to a separate file.

The diff depends on D15171 and D15183

Test Plan: make check

Reviewers: dhruba, haobo, kailiu, sdong

Reviewed By: kailiu

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15189
Move more functions from VersionSet to Version

Summary:
This moves functions:
* VersionSet::Finalize() -> Version::UpdateCompactionStats()
* VersionSet::UpdateFilesBySize() -> Version::UpdateFilesBySize()

The diff depends on D15189, D15183 and D15171

Test Plan: make check

Reviewers: kailiu, sdong, haobo, dhruba

Reviewed By: sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15201
Remove the unnecessary use of shared_ptr

Summary:
shared_ptr is slower than unique_ptr (which literally comes with no performance cost compare with raw pointers).
In memtable and memtable rep, we use shared_ptr when we'd actually should use unique_ptr.

According to igor's previous work, we are likely to make quite some performance gain from this diff.

Test Plan: make check

Reviewers: dhruba, igor, sdong, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15213
Merge branch 'master' into performance

Conflicts:
	Makefile
	db/db_impl.cc
	db/db_impl.h
	db/db_test.cc
	db/memtable.cc
	db/memtable.h
	db/version_edit.h
	db/version_set.cc
	include/rocksdb/options.h
	util/hash_skiplist_rep.cc
	util/options.cc
CompactionPicker

Summary:
This is a big one. This diff moves all the code related to picking compactions from VersionSet to new class CompactionPicker. Column families' compactions will be completely separate processes, so we need to have multiple CompactionPickers.

To make this easier to review, most of the code change is just copy/paste. There is also a small change not to use VersionSet::current_, but rather to take `Version* version` as a parameter. Most of the other code is exactly the same.

In future diffs, I will also make some improvements to CompactionPickers. I think the most important part will be encapsulating it better. Currently Version, VersionSet, Compaction and CompactionPicker are all friend classes, which makes it harder to change the implementation.

This diff depends on D15171, D15183, D15189 and D15201

Test Plan: `make check`

Reviewers: kailiu, sdong, dhruba, haobo

Reviewed By: kailiu

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15207
Remove compaction pointers

Summary: The only thing we do with compaction pointers is set them to some values, we never actually read them. I don't know what we used them for, but it doesn't look like we use them anymore.

Test Plan: make check

Reviewers: dhruba, haobo, kailiu, sdong

Reviewed By: kailiu

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15225
Fix some "make format" issue

Summary:
* make sure when some pre-check fails, the script won't halt immediately.
* change fburl to google's short url.
* Fix a bug in this script: now it checks the uncommitted code only.

Test Plan: Ran the script under differnet environments.

Reviewers: igor

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15231
Merge branch 'master' into performance

This patch merges master's changes on build_tools/format-diff.sh.
Conflicts:
	db/version_edit.cc
Allow callback to change size of existing value. Change return type of the callback function to an enum status to handle 3 cases.

Summary:
This diff fixes 2 hacks:
* The callback function can modify the existing value inplace, if the merged value fits within the existing buffer size. But currently the existing buffer size is not being modified. Now the callback recieves a int* allowing the size to be modified. Since size is encoded as a varint in the internal key for memtable. It might happen that the entire value might have be copied to the new location if the new size varint is smaller than the existing size varint.
* The callback function has 3 functionalities
    1. Modify existing buffer inplace, and update size correspondingly. Now to indicate that, Returns 1.
    2. Generate a new buffer indicating merged value. Returns 2.
    3. Fails to do either of above, based on whatever application logic. Returns 0.

Test Plan: Just make all for now. I'm adding another unit test to test each scenario.

Reviewers: dhruba, haobo

Reviewed By: haobo

CC: leveldb, sdong, kailiu, xinyaohu, sumeet, danguo

Differential Revision: https://reviews.facebook.net/D15195