Compaction Reads should read no more than compaction_readahead_size bytes, when set! (#5498)

Summary:
As a result of https://github.com/facebook/rocksdb/issues/5431 the compaction_readahead_size given by a user was not used exactly, the reason being the code behind readahead for user-read and compaction-read was unified in the above PR and the behavior for user-read is to read readahead_size+n bytes (see FilePrefetchBuffer::TryReadFromCache method). Before the unification the ReadaheadRandomAccessFileReader used compaction_readahead_size as it is.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5498

Test Plan:
Ran strace command : strace -e pread64 -f -T -t ./db_compaction_test --gtest_filter=DBCompactionTest.PartialManualCompaction

In the test the compaction_readahead_size was configured to 2MB and verified the pread syscall did indeed request 2MB. Before the change it was requesting more than 2MB.

Strace Output:
strace: Process 3798982 attached
Note: Google Test filter = DBCompactionTest.PartialManualCompaction
[==========] Running 1 test from 1 test case.
[----------] Global test environment set-up.
[----------] 1 test from DBCompactionTest
[ RUN      ] DBCompactionTest.PartialManualCompaction
strace: Process 3798983 attached
strace: Process 3798984 attached
strace: Process 3798985 attached
strace: Process 3798986 attached
strace: Process 3798987 attached
strace: Process 3798992 attached
[pid 3798987] 12:07:05 +++ exited with 0 +++
strace: Process 3798993 attached
[pid 3798993] 12:07:05 +++ exited with 0 +++
strace: Process 3798994 attached
strace: Process 3799008 attached
strace: Process 3799009 attached
[pid 3799008] 12:07:05 +++ exited with 0 +++
strace: Process 3799010 attached
[pid 3799009] 12:07:05 +++ exited with 0 +++
strace: Process 3799011 attached
[pid 3799010] 12:07:05 +++ exited with 0 +++
[pid 3799011] 12:07:05 +++ exited with 0 +++
strace: Process 3799012 attached
[pid 3799012] 12:07:05 +++ exited with 0 +++
strace: Process 3799013 attached
strace: Process 3799014 attached
[pid 3799013] 12:07:05 +++ exited with 0 +++
strace: Process 3799015 attached
[pid 3799014] 12:07:05 +++ exited with 0 +++
[pid 3799015] 12:07:05 +++ exited with 0 +++
strace: Process 3799016 attached
[pid 3799016] 12:07:05 +++ exited with 0 +++
strace: Process 3799017 attached
[pid 3799017] 12:07:05 +++ exited with 0 +++
strace: Process 3799019 attached
[pid 3799019] 12:07:05 +++ exited with 0 +++
strace: Process 3799020 attached
strace: Process 3799021 attached
[pid 3799020] 12:07:05 +++ exited with 0 +++
[pid 3799021] 12:07:05 +++ exited with 0 +++
strace: Process 3799022 attached
[pid 3799022] 12:07:05 +++ exited with 0 +++
strace: Process 3799023 attached
[pid 3799023] 12:07:05 +++ exited with 0 +++
strace: Process 3799047 attached
strace: Process 3799048 attached
[pid 3799047] 12:07:06 +++ exited with 0 +++
[pid 3799048] 12:07:06 +++ exited with 0 +++
[pid 3798994] 12:07:06 +++ exited with 0 +++
strace: Process 3799052 attached
[pid 3799052] 12:07:06 +++ exited with 0 +++
strace: Process 3799054 attached
strace: Process 3799069 attached
strace: Process 3799070 attached
[pid 3799069] 12:07:06 +++ exited with 0 +++
strace: Process 3799071 attached
[pid 3799070] 12:07:06 +++ exited with 0 +++
[pid 3799071] 12:07:06 +++ exited with 0 +++
strace: Process 3799072 attached
strace: Process 3799073 attached
[pid 3799072] 12:07:06 +++ exited with 0 +++
[pid 3799073] 12:07:06 +++ exited with 0 +++
strace: Process 3799074 attached
[pid 3799074] 12:07:06 +++ exited with 0 +++
strace: Process 3799075 attached
[pid 3799075] 12:07:06 +++ exited with 0 +++
strace: Process 3799076 attached
[pid 3799076] 12:07:06 +++ exited with 0 +++
strace: Process 3799077 attached
[pid 3799077] 12:07:06 +++ exited with 0 +++
strace: Process 3799078 attached
[pid 3799078] 12:07:06 +++ exited with 0 +++
strace: Process 3799079 attached
[pid 3799079] 12:07:06 +++ exited with 0 +++
strace: Process 3799080 attached
[pid 3799080] 12:07:06 +++ exited with 0 +++
strace: Process 3799081 attached
[pid 3799081] 12:07:06 +++ exited with 0 +++
strace: Process 3799082 attached
[pid 3799082] 12:07:06 +++ exited with 0 +++
strace: Process 3799083 attached
[pid 3799083] 12:07:06 +++ exited with 0 +++
strace: Process 3799086 attached
strace: Process 3799087 attached
[pid 3798984] 12:07:06 pread64(9, "\1\203W!\241QE\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 53, 11177) = 53 <0.000121>
[pid 3798984] 12:07:06 pread64(9, "\0\22\4rocksdb.properties\353Q\223\5\0\0\0\0\1\0\0"..., 38, 11139) = 38 <0.000106>
[pid 3798984] 12:07:06 pread64(9, "\0$\4rocksdb.block.based.table.ind"..., 664, 10475) = 664 <0.000081>
[pid 3798984] 12:07:06 pread64(9, "\0\v\3foo\2\7\0\0\0\0\0\0\0\270 \0\v\4foo\2\3\0\0\0\0\0\0\275"..., 74, 10401) = 74 <0.000138>
[pid 3798984] 12:07:06 pread64(11, "\1\203W!\241QE\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 53, 11177) = 53 <0.000097>
[pid 3798984] 12:07:06 pread64(11, "\0\22\4rocksdb.properties\353Q\223\5\0\0\0\0\1\0\0"..., 38, 11139) = 38 <0.000086>
[pid 3798984] 12:07:06 pread64(11, "\0$\4rocksdb.block.based.table.ind"..., 664, 10475) = 664 <0.000064>
[pid 3798984] 12:07:06 pread64(11, "\0\v\3foo\2\21\0\0\0\0\0\0\0\270 \0\v\4foo\2\r\0\0\0\0\0\0\275"..., 74, 10401) = 74 <0.000064>
[pid 3798984] 12:07:06 pread64(12, "\1\203W!\241QE\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 53, 11177) = 53 <0.000080>
[pid 3798984] 12:07:06 pread64(12, "\0\22\4rocksdb.properties\353Q\223\5\0\0\0\0\1\0\0"..., 38, 11139) = 38 <0.000090>
[pid 3798984] 12:07:06 pread64(12, "\0$\4rocksdb.block.based.table.ind"..., 664, 10475) = 664 <0.000059>
[pid 3798984] 12:07:06 pread64(12, "\0\v\3foo\2\33\0\0\0\0\0\0\0\270 \0\v\4foo\2\27\0\0\0\0\0\0\275"..., 74, 10401) = 74 <0.000065>
[pid 3798984] 12:07:06 pread64(13, "\1\203W!\241QE\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 53, 11177) = 53 <0.000070>
[pid 3798984] 12:07:06 pread64(13, "\0\22\4rocksdb.properties\353Q\223\5\0\0\0\0\1\0\0"..., 38, 11139) = 38 <0.000059>
[pid 3798984] 12:07:06 pread64(13, "\0$\4rocksdb.block.based.table.ind"..., 664, 10475) = 664 <0.000061>
[pid 3798984] 12:07:06 pread64(13, "\0\v\3foo\2%\0\0\0\0\0\0\0\270 \0\v\4foo\2!\0\0\0\0\0\0\275"..., 74, 10401) = 74 <0.000065>
[pid 3798984] 12:07:06 pread64(14, "\1\203W!\241QE\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 53, 11177) = 53 <0.000118>
[pid 3798984] 12:07:06 pread64(14, "\0\22\4rocksdb.properties\353Q\223\5\0\0\0\0\1\0\0"..., 38, 11139) = 38 <0.000093>
[pid 3798984] 12:07:06 pread64(14, "\0$\4rocksdb.block.based.table.ind"..., 664, 10475) = 664 <0.000050>
[pid 3798984] 12:07:06 pread64(14, "\0\v\3foo\2/\0\0\0\0\0\0\0\270 \0\v\4foo\2+\0\0\0\0\0\0\275"..., 74, 10401) = 74 <0.000082>
[pid 3798984] 12:07:06 pread64(15, "\1\203W!\241QE\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 53, 11177) = 53 <0.000080>
[pid 3798984] 12:07:06 pread64(15, "\0\22\4rocksdb.properties\353Q\223\5\0\0\0\0\1\0\0"..., 38, 11139) = 38 <0.000086>
[pid 3798984] 12:07:06 pread64(15, "\0$\4rocksdb.block.based.table.ind"..., 664, 10475) = 664 <0.000091>
[pid 3798984] 12:07:06 pread64(15, "\0\v\3foo\0029\0\0\0\0\0\0\0\270 \0\v\4foo\0025\0\0\0\0\0\0\275"..., 74, 10401) = 74 <0.000174>
[pid 3798984] 12:07:06 pread64(16, "\1\203W!\241QE\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 53, 11177) = 53 <0.000080>
[pid 3798984] 12:07:06 pread64(16, "\0\22\4rocksdb.properties\353Q\223\5\0\0\0\0\1\0\0"..., 38, 11139) = 38 <0.000093>
[pid 3798984] 12:07:06 pread64(16, "\0$\4rocksdb.block.based.table.ind"..., 664, 10475) = 664 <0.000194>
[pid 3798984] 12:07:06 pread64(16, "\0\v\3foo\2C\0\0\0\0\0\0\0\270 \0\v\4foo\2?\0\0\0\0\0\0\275"..., 74, 10401) = 74 <0.000086>
[pid 3798984] 12:07:06 pread64(17, "\1\203W!\241QE\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 53, 11177) = 53 <0.000079>
[pid 3798984] 12:07:06 pread64(17, "\0\22\4rocksdb.properties\353Q\223\5\0\0\0\0\1\0\0"..., 38, 11139) = 38 <0.000047>
[pid 3798984] 12:07:06 pread64(17, "\0$\4rocksdb.block.based.table.ind"..., 664, 10475) = 664 <0.000045>
[pid 3798984] 12:07:06 pread64(17, "\0\v\3foo\2M\0\0\0\0\0\0\0\270 \0\v\4foo\2I\0\0\0\0\0\0\275"..., 74, 10401) = 74 <0.000107>
[pid 3798983] 12:07:06 pread64(17, "\0\v\200\10foo\2P\0\0\0\0\0\0)U?MSg_)j(roFn($e"..., 2097152, 0) = 11230 <0.000091>
[pid 3798983] 12:07:06 pread64(17, "", 2085922, 11230) = 0 <0.000073>
[pid 3798983] 12:07:06 pread64(16, "\0\v\200\10foo\2F\0\0\0\0\0\0k[h3%.OPH_^:\\S7T&"..., 2097152, 0) = 11230 <0.000083>
[pid 3798983] 12:07:06 pread64(16, "", 2085922, 11230) = 0 <0.000078>
[pid 3798983] 12:07:06 pread64(15, "\0\v\200\10foo\2<\0\0\0\0\0\0+qToi_c{*S+4:N(:"..., 2097152, 0) = 11230 <0.000095>
[pid 3798983] 12:07:06 pread64(15, "", 2085922, 11230) = 0 <0.000067>
[pid 3798983] 12:07:06 pread64(14, "\0\v\200\10foo\0022\0\0\0\0\0\0%hw%OMa\"}9I609Q!B"..., 2097152, 0) = 11230 <0.000111>
[pid 3798983] 12:07:06 pread64(14, "", 2085922, 11230) = 0 <0.000093>
[pid 3798983] 12:07:06 pread64(13, "\0\v\200\10foo\2(\0\0\0\0\0\0p}Y&mu^DcaSGb2&nP"..., 2097152, 0) = 11230 <0.000128>
[pid 3798983] 12:07:06 pread64(13, "", 2085922, 11230) = 0 <0.000076>
[pid 3798983] 12:07:06 pread64(12, "\0\v\200\10foo\2\36\0\0\0\0\0\0YIyW#]oSs^6VHfB<`"..., 2097152, 0) = 11230 <0.000092>
[pid 3798983] 12:07:06 pread64(12, "", 2085922, 11230) = 0 <0.000073>
[pid 3798983] 12:07:06 pread64(11, "\0\v\200\10foo\2\24\0\0\0\0\0\0mfF8Jel/*Zf :-#s("..., 2097152, 0) = 11230 <0.000088>
[pid 3798983] 12:07:06 pread64(11, "", 2085922, 11230) = 0 <0.000067>
[pid 3798983] 12:07:06 pread64(9, "\0\v\200\10foo\2\n\0\0\0\0\0\0\\X'cjiHX)D,RSj1X!"..., 2097152, 0) = 11230 <0.000115>
[pid 3798983] 12:07:06 pread64(9, "", 2085922, 11230) = 0 <0.000073>
[pid 3798983] 12:07:06 pread64(8, "\1\315\5 \36\30\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 53, 754) = 53 <0.000098>
[pid 3798983] 12:07:06 pread64(8, "\0\22\3rocksdb.properties;\215\5\0\0\0\0\1\0\0\0"..., 37, 717) = 37 <0.000064>
[pid 3798983] 12:07:06 pread64(8, "\0$\4rocksdb.block.based.table.ind"..., 658, 59) = 658 <0.000074>
[pid 3798983] 12:07:06 pread64(8, "\0\v\2foo\1\0\0\0\0\0\0\0\0\31\0\0\0\0\1\0\0\0\0\212\216\222P", 29, 30) = 29 <0.000064>
[pid 3799086] 12:07:06 +++ exited with 0 +++
[pid 3799087] 12:07:06 +++ exited with 0 +++
[pid 3799054] 12:07:06 +++ exited with 0 +++
strace: Process 3799104 attached
[pid 3799104] 12:07:06 +++ exited with 0 +++
[       OK ] DBCompactionTest.PartialManualCompaction (757 ms)
[----------] 1 test from DBCompactionTest (758 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test case ran. (759 ms total)
[  PASSED  ] 1 test.
[pid 3798983] 12:07:06 +++ exited with 0 +++
[pid 3798984] 12:07:06 +++ exited with 0 +++
[pid 3798992] 12:07:06 +++ exited with 0 +++
[pid 3798986] 12:07:06 +++ exited with 0 +++
[pid 3798982] 12:07:06 +++ exited with 0 +++
[pid 3798985] 12:07:06 +++ exited with 0 +++
12:07:06 +++ exited with 0 +++

Differential Revision: D15948422

Pulled By: vjnadimpalli

fbshipit-source-id: 9b189d1e8675d290c7784e4b33e5d3b5761d2ac8

util/file_reader_writer.cc

@@ -797,9 +797,12 @@ bool FilePrefetchBuffer::TryReadFromCache(uint64_t offset, size_t n,
     if (readahead_size_ > 0) {
       assert(file_reader_ != nullptr);
       assert(max_readahead_size_ >= readahead_size_);
-
+      Status s;
-      Status s =
+      if (for_compaction) {
-          Prefetch(file_reader_, offset, n + readahead_size_, for_compaction);
+        s = Prefetch(file_reader_, offset, readahead_size_, for_compaction);
+      } else {
+        s = Prefetch(file_reader_, offset, n + readahead_size_, for_compaction);
+      }
       if (!s.ok()) {
         return false;
       }