Hi,

I wanted to get some input from the mailing list before making a JIRA and potential fixes. I'll touch the performance more on latter part, but there's one important question regarding the write latency metric recording place. Currently we measure the writeLatency (and metric write sampler..) in ColumnFamilyStore.apply() and this is also the metric we then replicate to Keyspace metrics etc.


This is an odd place for writeLatency. Not to mention it is in a hot-path of Memtable-modifications, but it also does not measure the real write latency, since it completely ignores the CommitLog latency in that same process. Is the intention really to measure Memtable-modification latency only or the actual write latencies?

Then the real issue.. this single metric is a cause of huge overhead in Memtable processing. There are several metrics / events in the CFS apply method, including metric sampler, storageHook reportWrite, colUpdateTimeDeltaHistogram and metric.writeLatency. These are not free at all when it comes to the processing. I made a small JMH benchmark here: https://gist.github.com/burmanm/b5b284bc9f1d410b1d635f6d3dac3ade that I'll be referring to.

The most offending of all these metrics is the writeLatency metric. What it does is update the latency in codahale's timer, doing a histogram update and then going through all the parent metrics also which update the keyspace writeLatency and globalWriteLatency. When measuring the performance of Memtable.put with parameter of 1 partition (to reduce the ConcurrentSkipListMap search speed impact - that's separate issue and takes a little bit longer to solve although I've started to prototype something..) on my machine I see 1.3M/s performance with the metric and when it is disabled the performance climbs to 4M/s. So the overhead for this single metric is ~2/3 of total performance. That's insane. My perf stats indicate that the CPU is starved as it can't get enough data in.

Removing the replication from TableMetrics to the Keyspace & global latencies in the write time (and doing this when metrics are requested instead) improves the performance to 2.1M/s on my machine. It's an improvement, but it's still huge amount. Even when we pressure the ConcurrentSkipListMap with 100 000 partitions in one active Memtable, the performance drops by about ~40% due to this metric, so it's never free.

i did not find any discussion replacing the metric processing with something faster, so has this been considered before? At least for these performance sensitive ones. The other issue is obviously the use of System.nanotime() which by itself is very slow (two System.nanotime() calls eat another ~1M/s from the performance)

My personal quick fix would be to move writeLatency to Keyspace.apply, change write time aggregates to read time processing (metrics are read less often than we write data) and maybe even reduce the nanotime -> currentTimeMillis (even given it's relative lack of precision). That is - if these metrics make any sense at all at CFS level? Maybe these should be measured from the network processing time (including all the deserializations and such) ? Especially if at some point the smarter threading / eventlooping changes go forward (in which case they might sleep at some "queue" for a while).

  - Micke


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