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Add comments to perf_context skip counters

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Summary: Document the skipped counters in perf context more clearly.

Test Plan: Comment only.

Reviewers: IslamAbdelRahman, yhchiang, MarkCallaghan

Reviewed By: MarkCallaghan

Subscribers: leveldb, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D55833
This commit is contained in:
sdong 2016-03-22 09:41:31 -07:00
parent 4e85b74790
commit 3996770d0b
1 changed files with 26 additions and 3 deletions
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29
include/rocksdb/perf_context.h
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@ -30,10 +30,33 @@ struct PerfContext {
uint64_t block_read_time; // total nanos spent on block reads
uint64_t block_checksum_time; // total nanos spent on block checksum
uint64_t block_decompress_time; // total nanos spent on block decompression
// total number of internal keys skipped over during iteration (overwritten or
// deleted, to be more specific, hidden by a put or delete of the same key)
// total number of internal keys skipped over during iteration.
// There are several reasons for it:
// 1. when calling Next(), iterator is in the position of the previous key,
// so that we'll need to skip it. It means this counter will always
// incrmented in Next().
// 2. when calling Next(), needs to skip internal entries for the previous
// key that are overwritten.
// 3. when calling Next(), Seek() or SeekToFirst(), after previous key
// before calling Next(), the seek key in Seek() or the beginning for
// SeekToFirst(), there may be one or more deleted keys before the next
// valid key that the operation should place the iterator to. We need
// to skip both of the tombstone and updates hidden by the tombstones. The
// tombstones are not included in this counter, while previous updates
// hidden by the tombstones will be included here.
// 4. symmetric cases for Prev() and SeekToLast()
// We sometimes also skip entries of more recent updates than the snapshot
// we read from, but it is not counted.
//
uint64_t internal_key_skipped_count;
// total number of deletes and single deletes skipped over during iteration
// Total number of deletes and single deletes skipped over during iteration
// When calling Next(), Seek() or SeekToFirst(), after previous key before
// calling Next(), the seek key in Seek() or the beginning for SeekToFirst(),
// there may be one or more deleted keys before the next valid key that the
// operation should place the iterator to. Every deleted key is counted once.
// We don't recount here if there are still older updates invalidated by the
// tombstones.
//
uint64_t internal_delete_skipped_count;
uint64_t get_snapshot_time; // total nanos spent on getting snapshot