Summary:
Deadlock: a memtable flush holds DB::mutex_ and calls ThreadLocalPtr::Scrape(), which locks ThreadLocalPtr mutex; meanwhile, a thread exit handler locks ThreadLocalPtr mutex and calls SuperVersionUnrefHandle, which tries to lock DB::mutex_.
This deadlock is hit all the time on our workload. It blocks our release.
In general, the problem is that ThreadLocalPtr takes an arbitrary callback and calls it while holding a lock on a global mutex. The same global mutex is (at least in some cases) locked by almost all ThreadLocalPtr methods, on any instance of ThreadLocalPtr. So, there'll be a deadlock if the callback tries to do anything to any instance of ThreadLocalPtr, or waits for another thread to do so.
So, probably the only safe way to use ThreadLocalPtr callbacks is to do only do simple and lock-free things in them.
This PR fixes the deadlock by making sure that local_sv_ never holds the last reference to a SuperVersion, and therefore SuperVersionUnrefHandle never has to do any nontrivial cleanup.
I also searched for other uses of ThreadLocalPtr to see if they may have similar bugs. There's only one other use, in transaction_lock_mgr.cc, and it looks fine.
Closes https://github.com/facebook/rocksdb/pull/3510
Reviewed By: sagar0
Differential Revision: D7005346
Pulled By: al13n321
fbshipit-source-id: 37575591b84f07a891d6659e87e784660fde815f
Summary:
Calling `std::vector::reserve()` causes memory to be reallocated and then data to be moved. It was called prior to adding every block. This reallocation could be done a huge amount of times, e.g., for users with large index blocks.
Instead, we can simply use `std::vector::emplace_back()` in such a way that preserves the no-memory-leak guarantee, while letting the vector decide when to reallocate space. Now I see reallocation/moving happen O(logN) times, rather than O(N) times, where N is the final size of vector.
Closes https://github.com/facebook/rocksdb/pull/3508
Differential Revision: D6994228
Pulled By: ajkr
fbshipit-source-id: ab7c11e13ff37c8c6c8249be7a79566a4068cd27
Summary:
- removed a few unneeded variables
- fused some variable declarations and their assignments
- fixed right-trimming code in string_util.cc to not underflow
- simplifed an assertion
- move non-nullptr check assertion before dereferencing of that pointer
- pass an std::string function parameter by const reference instead of by value (avoiding potential copy)
Closes https://github.com/facebook/rocksdb/pull/3507
Differential Revision: D7004679
Pulled By: sagar0
fbshipit-source-id: 52944952d9b56dfcac3bea3cd7878e315bb563c4
Summary:
We don't do fsync() after truncate in direct I/O writeable file (in fact we don't do any fsync ever). This can cause metadata not persistent to disk after the file is generated. We call it instead.
Closes https://github.com/facebook/rocksdb/pull/3500
Differential Revision: D6981482
Pulled By: siying
fbshipit-source-id: 7e2b591b7e5dd1b96fc0775515b8b9e6092980ef
Summary:
Now we suppress alignment UBSAN error as a whole. Suppressing 3-way CRC and murmurhash feels a better idea than turning off alignment check as a whole.
Closes https://github.com/facebook/rocksdb/pull/3495
Differential Revision: D6971273
Pulled By: siying
fbshipit-source-id: 080b59fed6df494b9f622ef7cb5d42d39e6a8cdf
Summary:
ForwardIterator::SVCleanup() sometimes didn't pin superversion when it was supposed to. See the added test for the scenario. Here's the ASAN output of the added test without the fix (using `COMPILE_WITH_ASAN=1 make`): https://pastebin.com/9rD0Ywws
Closes https://github.com/facebook/rocksdb/pull/3415
Differential Revision: D6817414
Pulled By: al13n321
fbshipit-source-id: bc80c44ea78a3a1fa885dfa448a26111f91afb24
Summary:
`ReadaheadRandomAccessFile` is used by iterators for file reads in several cases, like in compaction when `compaction_readahead_size > 0` or `use_direct_io_for_flush_and_compaction == true`, or in user iterator when `ReadOptions::readahead_size > 0`. `ReadaheadRandomAccessFile` maintains an internal buffer for readahead data. It assumes that, if the buffer's length is less than `ReadaheadRandomAccessFile::readahead_size_`, which is fixed in the constructor, then EOF has been reached so it doesn't try reading further.
Recently, d938226af4 started calling `RandomAccessFile::Prefetch` with various lengths: 8KB, 16KB, etc. When the `RandomAccessFile` is a `ReadaheadRandomAccessFile`, it triggers the above condition and incorrectly determines EOF. If a block is partially in the readahead buffer and EOF is incorrectly decided, the result is a truncated data block.
The problem is reproducible:
```
TEST_TMPDIR=/data/compaction_bench ./db_bench -benchmarks=fillrandom -write_buffer_size=1048576 -target_file_size_base=1048576 -block_size=18384 -use_direct_io_for_flush_and_compaction=true
...
put error: Corruption: truncated block read from /data/compaction_bench/dbbench/000014.sst offset 20245, expected 10143 bytes, got 8427
```
Closes https://github.com/facebook/rocksdb/pull/3454
Differential Revision: D6869405
Pulled By: ajkr
fbshipit-source-id: 87001c299e7600a37c0dcccbd0368e0954c929cf
Summary:
FilePrefetchBuffer::Prefetch is currently rounds the offset up which does not fit its new use cases in prefetching index/filter blocks, as it would skips over some the offsets that were requested to be prefetched. This patch rounds down instead.
Fixes#3180
Closes https://github.com/facebook/rocksdb/pull/3413
Differential Revision: D6816392
Pulled By: maysamyabandeh
fbshipit-source-id: 3aaeaf59c55d72b61dacfae6d4a8e65eccb3c553
Summary:
Currently, the only way to close an open DB is to destroy the DB
object. There is no way for the caller to know the status. In one
instance, the destructor encountered an error due to failure to
close a log file on HDFS. In order to prevent silent failures, we add
DB::Close() that calls CloseImpl() which must be implemented by its
descendants.
The main failure point in the destructor is closing the log file. This
patch also adds a Close() entry point to Logger in order to get status.
When DBOptions::info_log is allocated and owned by the DBImpl, it is
explicitly closed by DBImpl::CloseImpl().
Closes https://github.com/facebook/rocksdb/pull/3348
Differential Revision: D6698158
Pulled By: anand1976
fbshipit-source-id: 9468e2892553eb09c4c41b8723f590c0dbd8ab7d
Summary:
This fixes the following warnings when compiled with GCC7:
util/transaction_test_util.cc: In static member function ‘static rocksdb::Status rocksdb::RandomTransactionInserter::DBGet(rocksdb::DB*, rocksdb::Transaction*, rocksdb::ReadOptions&, uint16_t, uint64_t, bool, uint64_t*, std::__cxx11::string*, bool*)’:
util/transaction_test_util.cc:75:8: error: ‘snprintf’ output may be truncated before the last format character [-Werror=format-truncation=]
Status RandomTransactionInserter::DBGet(
^~~~~~~~~~~~~~~~~~~~~~~~~
util/transaction_test_util.cc:84:11: note: ‘snprintf’ output between 5 and 6 bytes into a destination of size 5
snprintf(prefix_buf, sizeof(prefix_buf), "%.4u", set_i + 1);
~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
util/transaction_test_util.cc: In static member function ‘static rocksdb::Status rocksdb::RandomTransactionInserter::Verify(rocksdb::DB*, uint16_t, uint64_t, bool, rocksdb::Random64*)’:
util/transaction_test_util.cc:245:8: error: ‘snprintf’ output may be truncated before the last format character [-Werror=format-truncation=]
Status RandomTransactionInserter::Verify(DB* db, uint16_t num_sets,
^~~~~~~~~~~~~~~~~~~~~~~~~
util/transaction_test_util.cc:268:13: note: ‘snprintf’ output between 5 and 6 bytes into a destination of size 5
snprintf(prefix_buf, sizeof(prefix_buf), "%.4u", set_i + 1);
~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Closes https://github.com/facebook/rocksdb/pull/3295
Differential Revision: D6609411
Pulled By: maysamyabandeh
fbshipit-source-id: 33f0add471056eb59db2f8bd4366e6dfbb1a187d
Summary:
**# Summary**
RocksDB uses SSE crc32 intrinsics to calculate the crc32 values but it does it in single way fashion (not pipelined on single CPU core). Intel's whitepaper () published an algorithm that uses 3-way pipelining for the crc32 intrinsics, then use pclmulqdq intrinsic to combine the values. Because pclmulqdq has overhead on its own, this algorithm will show perf gains on buffers larger than 216 bytes, which makes RocksDB a perfect user, since most of the buffers RocksDB call crc32c on is over 4KB. Initial db_bench show tremendous CPU gain.
This change uses the 3-way SSE algorithm by default. The old SSE algorithm is now behind a compiler tag NO_THREEWAY_CRC32C. If user compiles the code with NO_THREEWAY_CRC32C=1 then the old SSE Crc32c algorithm would be used. If the server does not have SSE4.2 at the run time the slow way (Non SSE) will be used.
**# Performance Test Results**
We ran the FillRandom and ReadRandom benchmarks in db_bench. ReadRandom is the point of interest here since it calculates the CRC32 for the in-mem buffers. We did 3 runs for each algorithm.
Before this change the CRC32 value computation takes about 11.5% of total CPU cost, and with the new 3-way algorithm it reduced to around 4.5%. The overall throughput also improved from 25.53MB/s to 27.63MB/s.
1) ReadRandom in db_bench overall metrics
PER RUN
Algorithm | run | micros/op | ops/sec |Throughput (MB/s)
3-way | 1 | 4.143 | 241387 | 26.7
3-way | 2 | 3.775 | 264872 | 29.3
3-way | 3 | 4.116 | 242929 | 26.9
FastCrc32c|1 | 4.037 | 247727 | 27.4
FastCrc32c|2 | 4.648 | 215166 | 23.8
FastCrc32c|3 | 4.352 | 229799 | 25.4
AVG
Algorithm | Average of micros/op | Average of ops/sec | Average of Throughput (MB/s)
3-way | 4.01 | 249,729 | 27.63
FastCrc32c | 4.35 | 230,897 | 25.53
2) Crc32c computation CPU cost (inclusive samples percentage)
PER RUN
Implementation | run | TotalSamples | Crc32c percentage
3-way | 1 | 4,572,250,000 | 4.37%
3-way | 2 | 3,779,250,000 | 4.62%
3-way | 3 | 4,129,500,000 | 4.48%
FastCrc32c | 1 | 4,663,500,000 | 11.24%
FastCrc32c | 2 | 4,047,500,000 | 12.34%
FastCrc32c | 3 | 4,366,750,000 | 11.68%
**# Test Plan**
make -j64 corruption_test && ./corruption_test
By default it uses 3-way SSE algorithm
NO_THREEWAY_CRC32C=1 make -j64 corruption_test && ./corruption_test
make clean && DEBUG_LEVEL=0 make -j64 db_bench
make clean && DEBUG_LEVEL=0 NO_THREEWAY_CRC32C=1 make -j64 db_bench
Closes https://github.com/facebook/rocksdb/pull/3173
Differential Revision: D6330882
Pulled By: yingsu00
fbshipit-source-id: 8ec3d89719533b63b536a736663ca6f0dd4482e9
Summary:
added `ThreadType::BOTTOM_PRIORITY` which is used in the `ThreadStatus` object to indicate the thread is used for bottom-pri compactions. Previously there was a bug where we mislabeled such threads as `ThreadType::LOW_PRIORITY`.
Closes https://github.com/facebook/rocksdb/pull/3270
Differential Revision: D6559428
Pulled By: ajkr
fbshipit-source-id: 96b1a50a9c19492b1a5fd1b77cf7061a6f9f1d1c
Summary:
There were a few places where MSVC's implicit truncation warnings were getting triggered, which was causing the MSVC build to fail due to warnings being treated as errors. This resolves the issues by making the truncations in some places explicit, and by making it so there are no truncations of literals.
Fixes#3239
Supersedes #3259
Closes https://github.com/facebook/rocksdb/pull/3273
Reviewed By: yiwu-arbug
Differential Revision: D6569204
Pulled By: Orvid
fbshipit-source-id: c188cf1cf98d9acb6d94b71875041cc81f8ff088
Summary:
DeleteScheduler::MarkAsTrash() don't handle existing .trash files correctly
This cause rocksdb to not being able to delete existing .trash files on restart
Closes https://github.com/facebook/rocksdb/pull/3261
Differential Revision: D6548003
Pulled By: IslamAbdelRahman
fbshipit-source-id: c3800639412e587a690062c63076a5a08881e0e6
Summary:
I started adding gflags support for cmake on linux and got frustrated that I'd need to duplicate the build_detect_platform logic, which determines namespace based on attempting compilation. We can do it differently -- use the GFLAGS_NAMESPACE macro if available, and if not, that indicates it's an old gflags version without configurable namespace so we can simply hardcode "google".
Closes https://github.com/facebook/rocksdb/pull/3212
Differential Revision: D6456973
Pulled By: ajkr
fbshipit-source-id: 3e6d5bde3ca00d4496a120a7caf4687399f5d656
Summary:
Allow users to configure the trash-to-DB size ratio limit, so
that ratelimits for deletes can be enforced even when larger portions of
the database are being deleted.
Closes https://github.com/facebook/rocksdb/pull/3158
Differential Revision: D6304897
Pulled By: gdavidsson
fbshipit-source-id: a28dd13059ebab7d4171b953ed91ce383a84d6b3
Summary:
This confused some users who were getting compression type from the logs.
Closes https://github.com/facebook/rocksdb/pull/3153
Differential Revision: D6294964
Pulled By: ajkr
fbshipit-source-id: 3c813376d33682dc6ccafc9a78df1a2e2528985e
Summary:
The current implementation of PinnableSlice move assignment have an issue #3163. We are moving away from it instead of try to get the move assignment right, since it is too tricky.
Closes https://github.com/facebook/rocksdb/pull/3164
Differential Revision: D6319201
Pulled By: yiwu-arbug
fbshipit-source-id: 8f3279021f3710da4a4caa14fd238ed2df902c48
Summary:
After move assignment, we need to re-initialized the moved PinnableSlice.
Also update blob_db_impl.cc to not reuse the moved PinnableSlice since it is supposed to be in an undefined state after move.
Closes https://github.com/facebook/rocksdb/pull/3127
Differential Revision: D6238585
Pulled By: yiwu-arbug
fbshipit-source-id: bd99f2e37406c4f7de160c7dee6a2e8126bc224e
Summary:
util/concurrent_arena.h:
CID 1396145 (#1 of 1): Uninitialized pointer field (UNINIT_CTOR)
2. uninit_member: Non-static class member free_begin_ is not initialized in this constructor nor in any functions that it calls.
94 Shard() : allocated_and_unused_(0) {}
util/dynamic_bloom.cc:
1. Condition hash_func == NULL, taking true branch.
CID 1322821 (#1 of 1): Uninitialized pointer field (UNINIT_CTOR)
3. uninit_member: Non-static class member data_ is not initialized in this constructor nor in any functions that it calls.
47 hash_func_(hash_func == nullptr ? &BloomHash : hash_func) {}
48
util/file_reader_writer.h:
204 private:
205 AlignedBuffer buffer_;
member_not_init_in_gen_ctor: The compiler-generated constructor for this class does not initialize buffer_offset_.
206 uint64_t buffer_offset_;
CID 1418246 (#1 of 1): Uninitialized scalar field (UNINIT_CTOR)
member_not_init_in_gen_ctor: The compiler-generated constructor for this class does not initialize buffer_len_.
207 size_t buffer_len_;
208};
util/thread_local.cc:
341#endif
CID 1322795 (#1 of 1): Uninitialized scalar field (UNINIT_CTOR)
3. uninit_member: Non-static class member pthread_key_ is not initialized in this constructor nor in any functions that it calls.
342}
40struct ThreadData {
2. uninit_member: Non-static class member next is not initialized in this constructor nor in any functions that it calls.
CID 1400668 (#1 of 1): Uninitialized pointer field (UNINIT_CTOR)
4. uninit_member: Non-static class member prev is not initialized in this constructor nor in any functions that it calls.
41 explicit ThreadData(ThreadLocalPtr::StaticMeta* _inst) : entries(), inst(_inst) {}
42 std::vector<Entry> entries;
1. member_decl: Class member declaration for next.
43 ThreadData* next;
3. member_decl: Class member declaration for prev.
44 ThreadData* prev;
45 ThreadLocalPtr::StaticMeta* inst;
46};
Closes https://github.com/facebook/rocksdb/pull/3123
Differential Revision: D6233566
Pulled By: sagar0
fbshipit-source-id: aa2068790ea69787a0035c0db39d59b0c25108db
Summary:
Instead of using samples directly, we now support passing the samples through zstd's dictionary generator when `CompressionOptions::zstd_max_train_bytes` is set to nonzero. If set to zero, we will use the samples directly as the dictionary -- same as before.
Note this is the first step of #2987, extracted into a separate PR per reviewer request.
Closes https://github.com/facebook/rocksdb/pull/3057
Differential Revision: D6116891
Pulled By: ajkr
fbshipit-source-id: 70ab13cc4c734fa02e554180eed0618b75255497
Summary:
SstFileManager move files that need to be deleted into a trash directory.
Deprecate this behaviour and instead add ".trash" extension to files that need to be deleted
Closes https://github.com/facebook/rocksdb/pull/2970
Differential Revision: D5976805
Pulled By: IslamAbdelRahman
fbshipit-source-id: 27374ece4315610b2792c30ffcd50232d4c9a343
Summary:
ColumnFamilyOptions::compaction_options_fifo and all its sub-fields can be set dynamically now.
Some of the ways in which the fifo compaction options can be set are:
- `SetOptions({{"compaction_options_fifo", "{max_table_files_size=1024}"}})`
- `SetOptions({{"compaction_options_fifo", "{ttl=600;}"}})`
- `SetOptions({{"compaction_options_fifo", "{max_table_files_size=1024;ttl=600;}"}})`
- `SetOptions({{"compaction_options_fifo", "{max_table_files_size=51;ttl=49;allow_compaction=true;}"}})`
Most of the code has been made generic enough so that it could be reused later to make universal options (and other such nested defined-types) dynamic with very few lines of parsing/serializing code changes.
Introduced a few new functions like `ParseStruct`, `SerializeStruct` and `GetStringFromStruct`.
The duplicate code in `GetStringFromDBOptions` and `GetStringFromColumnFamilyOptions` has been moved into `GetStringFromStruct`. So they become just simple wrappers now.
Closes https://github.com/facebook/rocksdb/pull/3006
Differential Revision: D6058619
Pulled By: sagar0
fbshipit-source-id: 1e8f78b3374ca5249bb4f3be8a6d3bb4cbc52f92
Summary:
When I impl my own comparator, and build in release mode.
The following compile error occurs.
undefined reference to `typeinfo for rocksdb::Comparator'
This fix allows users build with RTTI off when has their own comparator.
Closes https://github.com/facebook/rocksdb/pull/3008
Differential Revision: D6077354
Pulled By: yiwu-arbug
fbshipit-source-id: 914c26dbab72f0ad1f0e15f8666a3fb2f10bfed8
Summary:
it turns out that, with older GCC shipped from centos7, the SSE42
intrinsics are not available even with "target" specified. so we
need to pass "-msse42" for checking compiler's sse4.2 support and
for building crc32c.cc which uses sse4.2 intrinsics for crc32.
Signed-off-by: Kefu Chai <tchaikov@gmail.com>
Closes https://github.com/facebook/rocksdb/pull/2950
Differential Revision: D6032298
Pulled By: siying
fbshipit-source-id: 124c946321043661b3fb0a70b6cdf4c9c5126ab4
Summary:
Dynamic adjustment of rate limit according to demand for background I/O. It increases by a factor when limiter is drained too frequently, and decreases by the same factor when limiter is not drained frequently enough. The parameters for this behavior are fixed in `GenericRateLimiter::Tune`. Other changes:
- make rate limiter's `Env*` configurable for testing
- track num drain intervals in RateLimiter so we don't have to rely on stats, which may be shared across different DB instances from the ones that share the RateLimiter.
Closes https://github.com/facebook/rocksdb/pull/2899
Differential Revision: D5858704
Pulled By: ajkr
fbshipit-source-id: cc2bac30f85e7f6fd63655d0a6732ef9ed7403b1
Summary:
Previously the thread pool might be non-empty after joining since concurrent submissions could spawn new threads. This problem didn't affect our background flush/compaction thread pools because the `shutting_down_` flag prevented new jobs from being submitted during/after joining. But I wanted to be able to reuse the `ThreadPool` without such external synchronization.
Closes https://github.com/facebook/rocksdb/pull/2953
Differential Revision: D5951920
Pulled By: ajkr
fbshipit-source-id: 0efec7d0056d36d1338367da75e8b0c089bbc973
Summary:
Merging iterator invokes InternalKeyComparator.Compare() frequently to heap merge. By making InternalKeyComparator final and merging iterator to directly use InternalKeyComparator rather than through Iterator interface, we can give compiler a choice to avoid one more virtual function call if possible. I ran readseq benchmark in memory-only use case to make sure the performance at least doesn't regress.
I have to disable the final key word in debug build, as a hack test class depends on overriding the class.
Closes https://github.com/facebook/rocksdb/pull/2860
Differential Revision: D5800461
Pulled By: siying
fbshipit-source-id: ab876f22a09bb5c560740911412336e0e25ccb53
Summary:
if we enable SSE42 globally when compiling the tree for preparing a
portable binary, which could be running on CPU w/o SSE42 instructions
even the GCC on the building host is able to emit SSE42 code, this leads
to illegal instruction errors on machines not supporting SSE42. to solve
this problem, crc32 detects the supported instruction at runtime, and
selects the supported CRC32 implementation according to the result of
`cpuid`. but intrinics like "_mm_crc32_u64()" will not be available
unless the "target" machine is appropriately specified in the command
line, like "-msse42", or using the "target" attribute.
we could pass "-msse42" only when compiling crc32c.cc, and allow the
compiler to generate the SSE42 instructions, but we are still at the
risk of executing illegal instructions on machines does not support
SSE42 if the compiler emits code that is not guarded by our runtime
detection. and we need to do the change in both Makefile and CMakefile.
or, we can use GCC's "target" attribute to enable the machine specific
instructions on certain function. in this way, we have finer grained
control of the used "target". and no need to change the makefiles. so
we don't need to duplicate the changes on both makefile and cmake as
the previous approach.
this problem surfaces when preparing a package for GNU/Linux distribution,
and we only applies to optimization for SSE42, so using a feature
only available on GCC/Clang is not that formidable.
Closes https://github.com/facebook/rocksdb/pull/2807
Differential Revision: D5786084
Pulled By: siying
fbshipit-source-id: bca5c0f877b8d6fb55f58f8f122254a26422843d
Summary:
Right now, if direct I/O is enabled, prefetching the last 512KB cannot be applied, except compaction inputs or readahead is enabled for iterators. This can create a lot of I/O for HDD cases. To solve the problem, the 512KB is prefetched in block based table if direct I/O is enabled. The prefetched buffer is passed in totegher with random access file reader, so that we try to read from the buffer before reading from the file. This can be extended in the future to support flexible user iterator readahead too.
Closes https://github.com/facebook/rocksdb/pull/2708
Differential Revision: D5593091
Pulled By: siying
fbshipit-source-id: ee36ff6d8af11c312a2622272b21957a7b5c81e7
Summary:
When we had a single thread pool for compactions, a thread could be busy for a long time (minutes) executing a compaction involving the bottom level. In multi-instance setups, the entire thread pool could be consumed by such bottom-level compactions. Then, top-level compactions (e.g., a few L0 files) would be blocked for a long time ("head-of-line blocking"). Such top-level compactions are critical to prevent compaction stalls as they can quickly reduce number of L0 files / sorted runs.
This diff introduces a bottom-priority queue for universal compactions including the bottom level. This alleviates the head-of-line blocking situation for fast, top-level compactions.
- Added `Env::Priority::BOTTOM` thread pool. This feature is only enabled if user explicitly configures it to have a positive number of threads.
- Changed `ThreadPoolImpl`'s default thread limit from one to zero. This change is invisible to users as we call `IncBackgroundThreadsIfNeeded` on the low-pri/high-pri pools during `DB::Open` with values of at least one. It is necessary, though, for bottom-pri to start with zero threads so the feature is disabled by default.
- Separated `ManualCompaction` into two parts in `PrepickedCompaction`. `PrepickedCompaction` is used for any compaction that's picked outside of its execution thread, either manual or automatic.
- Forward universal compactions involving last level to the bottom pool (worker thread's entry point is `BGWorkBottomCompaction`).
- Track `bg_bottom_compaction_scheduled_` so we can wait for bottom-level compactions to finish. We don't count them against the background jobs limits. So users of this feature will get an extra compaction for free.
Closes https://github.com/facebook/rocksdb/pull/2580
Differential Revision: D5422916
Pulled By: ajkr
fbshipit-source-id: a74bd11f1ea4933df3739b16808bb21fcd512333
