Optionally construct Post Processing Info map in MemTableInserter

[yuslepukhin]

MemTableInserter default constructs Post processing info
std::map. However, on Windows with 2015 STL the default
constructed map still dynamically allocates one node
which shows up on a profiler and we loose ~40% throughput
on fillrandom benchmark.
Solution: declare a map as std::aligned storage and optionally
construct.

This addresses #1976

Before:

Initializing RocksDB Options from command-line flags
DB path: [k:\data\BulkLoadRandom_10M_fillonly]
fillrandom : 2.775 micros/op 360334 ops/sec; 280.4 MB/s
Microseconds per write:
Count: 10000000 Average: 2.7749 StdDev: 39.92
Min: 1 Median: 2.0826 Max: 26051
Percentiles: P50: 2.08 P75: 2.55 P99: 3.55 P99.9: 9.58 P99.99: 51.5**6

After:

Initializing RocksDB Options from command-line flags
DB path: [k:\data\BulkLoadRandom_10M_fillonly]
fillrandom : 1.794 micros/op 557284 ops/sec; 433.7 MB/s
Microseconds per write:
Count: 10000000 Average: 1.7940 StdDev: 45.46
Min: 0 Median: 1.0812 Max: 23620
Percentiles: P50: 1.08 P75: 1.57 P99: 2.81 P99.9: 5.39 P99.99: 17.36

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[IslamAbdelRahman]

Thanks!
will using std::unique_ptrstd::map have the same effect ?

[siying]

@IslamAbdelRahman unique_ptr is a pointer, so the map itself needs an extra allocation.

[yuslepukhin]

@IslamAbdelRahman @siying is correct. We'd need an extra allocation for map itself. Preserving space reserved for map inside the inserter gives us the same cost-less stack allocation.

[yuslepukhin]

@siying I do not know what's wrong with Travis

[siying]

@yuslepukhin what we can do?

[siying]

@yuslepukhin failing for our internal CI too:

db/write_batch.cc:789:17: error: expected type-specifier
   PostMapType = std::aligned_storage_t<sizeof(MemPostInfoMap)>;
                 ^
db/write_batch.cc:790:3: error: ‘PostMapType’ does not name a type
   PostMapType mem_post_info_map_;
   ^
db/write_batch.cc: In member function ‘rocksdb::MemTableInserter::MemPostInfoMap& rocksdb::MemTableInserter::GetPostMap()’:
db/write_batch.cc:797:13: error: ‘mem_post_info_map_’ was not declared in this scope
       new (&mem_post_info_map_) MemPostInfoMap();
             ^
db/write_batch.cc:800:48: error: ‘mem_post_info_map_’ was not declared in this scope
     return *reinterpret_cast<MemPostInfoMap*>(&mem_post_info_map_);
                                                ^
db/write_batch.cc: In destructor ‘virtual rocksdb::MemTableInserter::~MemTableInserter()’:
db/write_batch.cc:828:11: error: ‘mem_post_info_map_’ was not declared in this scope
         (&mem_post_info_map_)->~MemPostInfoMap();
           ^

[yuslepukhin]

@siying I have included <type_traits> for std::std::aligned_storage_t,
It looks like std::std::aligned_storage_t is not defined, in which case we will need to replace it with std::std::aligned_storage<sizeof(MemPostInfoMap)>::type

Will post an update

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@yuslepukhin updated the pull request - view changes - changes since last import

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