3 Commits (0a1aed4e717e68c4b6dbf3c21e9cc04ebe5b5171)

Author	SHA1	Message	Date
Peter Dillinger	d5a46c40e5	Pin CacheEntryStatsCollector to fix performance bug (#8385) ... Summary: If the block Cache is full with strict_capacity_limit=false, then our CacheEntryStatsCollector could be immediately evicted on release, so iterating through column families with shared block cache could trigger re-scan for each CF. This change fixes that problem by pinning the CacheEntryStatsCollector from InternalStats so that it's not evicted. I had originally thought that this object could participate in LRU like everything else, but even though a re-load+re-scan only touches memory, it can be orders of magnitude more expensive than other cache misses. One service in Facebook has scans that take ~20s over 100GB block cache that is mostly 4KB entries. (The up-side of this bug and https://github.com/facebook/rocksdb/issues/8369 is that we had a natural experiment on the effect on some service metrics even with block cache scans running continuously in the background--a kind of worst case scenario. Metrics like latency were not affected enough to trigger warnings.) Other smaller fixes: 20s is already a sizable portion of 600s stats dump period, or 180s default max age to force re-scan, so added logic to ensure that (for each block cache) we don't spend more than 0.2% of our background thread time scanning it. Nevertheless, "foreground" requests for cache entry stats (calls to `db->GetMapProperty(DB::Properties::kBlockCacheEntryStats)`) are permitted to consume more CPU. Renamed field to cache_entry_stats_ to match code style. This change is intended for patching in 6.21 release. Pull Request resolved: https://github.com/facebook/rocksdb/pull/8385 Test Plan: unit test expanded to cover new logic (detect regression), some manual testing with db_bench Reviewed By: ajkr Differential Revision: D29042759 Pulled By: pdillinger fbshipit-source-id: 236faa902397f50038c618f50fbc8cf3f277308c	4 years ago
Peter Dillinger	2f93a3b809	Fix a major performance bug in 6.21 for cache entry stats (#8369) ... Summary: In final polishing of https://github.com/facebook/rocksdb/issues/8297 (after most manual testing), I broke my own caching layer by sanitizing an input parameter with std::min(0, x) instead of std::max(0, x). I resisted unit testing the timing part of the result caching because historically, these test are either flaky or difficult to write, and this was not a correctness issue. This bug is essentially unnoticeable with a small number of column families but can explode background work with a large number of column families. This change fixes the logical error, removes some unnecessary related optimization, and adds mock time/sleeps to the unit test to ensure we can cache hit within the age limit. Pull Request resolved: https://github.com/facebook/rocksdb/pull/8369 Test Plan: added time testing logic to existing unit test Reviewed By: ajkr Differential Revision: D28950892 Pulled By: pdillinger fbshipit-source-id: e79cd4ff3eec68fd0119d994f1ed468c38026c3b	4 years ago
Peter Dillinger	311a544c2a	Use deleters to label cache entries and collect stats (#8297) ... Summary: This change gathers and publishes statistics about the kinds of items in block cache. This is especially important for profiling relative usage of cache by index vs. filter vs. data blocks. It works by iterating over the cache during periodic stats dump (InternalStats, stats_dump_period_sec) or on demand when DB::Get(Map)Property(kBlockCacheEntryStats), except that for efficiency and sharing among column families, saved data from the last scan is used when the data is not considered too old. The new information can be seen in info LOG, for example: Block cache LRUCache@0x7fca62229330 capacity: 95.37 MB collections: 8 last_copies: 0 last_secs: 0.00178 secs_since: 0 Block cache entry stats(count,size,portion): DataBlock(7092,28.24 MB,29.6136%) FilterBlock(215,867.90 KB,0.888728%) FilterMetaBlock(2,5.31 KB,0.00544%) IndexBlock(217,180.11 KB,0.184432%) WriteBuffer(1,256.00 KB,0.262144%) Misc(1,0.00 KB,0%) And also through DB::GetProperty and GetMapProperty (here using ldb just for demonstration): $ ./ldb --db=/dev/shm/dbbench/ get_property rocksdb.block-cache-entry-stats rocksdb.block-cache-entry-stats.bytes.data-block: 0 rocksdb.block-cache-entry-stats.bytes.deprecated-filter-block: 0 rocksdb.block-cache-entry-stats.bytes.filter-block: 0 rocksdb.block-cache-entry-stats.bytes.filter-meta-block: 0 rocksdb.block-cache-entry-stats.bytes.index-block: 178992 rocksdb.block-cache-entry-stats.bytes.misc: 0 rocksdb.block-cache-entry-stats.bytes.other-block: 0 rocksdb.block-cache-entry-stats.bytes.write-buffer: 0 rocksdb.block-cache-entry-stats.capacity: 8388608 rocksdb.block-cache-entry-stats.count.data-block: 0 rocksdb.block-cache-entry-stats.count.deprecated-filter-block: 0 rocksdb.block-cache-entry-stats.count.filter-block: 0 rocksdb.block-cache-entry-stats.count.filter-meta-block: 0 rocksdb.block-cache-entry-stats.count.index-block: 215 rocksdb.block-cache-entry-stats.count.misc: 1 rocksdb.block-cache-entry-stats.count.other-block: 0 rocksdb.block-cache-entry-stats.count.write-buffer: 0 rocksdb.block-cache-entry-stats.id: LRUCache@0x7f3636661290 rocksdb.block-cache-entry-stats.percent.data-block: 0.000000 rocksdb.block-cache-entry-stats.percent.deprecated-filter-block: 0.000000 rocksdb.block-cache-entry-stats.percent.filter-block: 0.000000 rocksdb.block-cache-entry-stats.percent.filter-meta-block: 0.000000 rocksdb.block-cache-entry-stats.percent.index-block: 2.133751 rocksdb.block-cache-entry-stats.percent.misc: 0.000000 rocksdb.block-cache-entry-stats.percent.other-block: 0.000000 rocksdb.block-cache-entry-stats.percent.write-buffer: 0.000000 rocksdb.block-cache-entry-stats.secs_for_last_collection: 0.000052 rocksdb.block-cache-entry-stats.secs_since_last_collection: 0 Solution detail - We need some way to flag what kind of blocks each entry belongs to, preferably without changing the Cache API. One of the complications is that Cache is a general interface that could have other users that don't adhere to whichever convention we decide on for keys and values. Or we would pay for an extra field in the Handle that would only be used for this purpose. This change uses a back-door approach, the deleter, to indicate the "role" of a Cache entry (in addition to the value type, implicitly). This has the added benefit of ensuring proper code origin whenever we recognize a particular role for a cache entry; if the entry came from some other part of the code, it will use an unrecognized deleter, which we simply attribute to the "Misc" role. An internal API makes for simple instantiation and automatic registration of Cache deleters for a given value type and "role". Another internal API, CacheEntryStatsCollector, solves the problem of caching the results of a scan and sharing them, to ensure scans are neither excessive nor redundant so as not to harm Cache performance. Because code is added to BlocklikeTraits, it is pulled out of block_based_table_reader.cc into its own file. This is a reformulation of https://github.com/facebook/rocksdb/issues/8276, without the type checking option (could still be added), and with actual stat gathering. Pull Request resolved: https://github.com/facebook/rocksdb/pull/8297 Test Plan: manual testing with db_bench, and a couple of basic unit tests Reviewed By: ltamasi Differential Revision: D28488721 Pulled By: pdillinger fbshipit-source-id: 472f524a9691b5afb107934be2d41d84f2b129fb	4 years ago