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HBase >> mail # dev >> SILT - nice keyvalue store paper


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RE: SILT - nice keyvalue store paper
Very nice experiment, Akash.  Keep getting your hands dirty and digging!  :)

I think your results might change if you bump the test up to 1000 threads or so.  100 threads can still perform okay when there's a global lock but the contention at 1000 threads will kill you and that's when CSLM should do much better.  (1000 handler threads is approx. what I run with on RS in prod).  Though I am a bit surprised that at 100 threads the TreeMap was significantly faster.  Your inconsistent results are a bit odd, you might try an order of magnitude more operations per thread.  You might also gather some statistics about tree size and per operation latency.

I've done some isolated CSLM benchmarks in the past and have never been able to reproduce any of the slowness people suggest.  I recall trying some impractically large MemStores and everything still being quite fast.

Over in Cassandra, I believe they have a two-level CSLM with the first map key being the row and then the columns for each row in their own CSLM.  I've been told this is somewhat of a pain point for them.  And keep in mind they have one shard/region per node and we generally have several smaller MemStores on each node (tens to thousands).  Not sure we would want to try that.  There could be some interesting optimizations if you had very specific issues, like if you had a ton of reads to MemStore and not many writes you could keep some kind of mirrored hashmap.

And for writes, the WAL is definitely the latency bottleneck.  But if you are doing lots of small operations, so your WALEdits are not large, and with some of the HLog batching features going in to trunk, you end up with hundreds of requests per HLog sync.  And although the syncs are higher latency, with batching you end up getting high throughput.  And the bottleneck shifts.

Each sync will take approx. 1-5ms, so let's say 250 requests per HLog sync batch, 4ms per sync, so 62.5k req/sec.  (62.5k * 100 bytes/req = 600K/sec, very reasonable).  If you're mixing in reads as well (or if you're doing increments which do a read and write), then this adds to the CPU usage and contention without adding to HLog throughput.

All of a sudden the bottleneck becomes CPU/contention and not HLog latency or throughput.  Highly concurrent increments/counters with a largely in-memory dataset can easily be CPU bottlenecked.

For one specific application Dhruba and I worked on, we made some good improvements in CPU efficiency by reducing the number of operations and increasing efficiency on the CSLM.  Doing things like always taking a tailMap and working from that instead of starting at the root node, using an iterator() and taking advantage of the available remove() semantics, or simply just mutating things that are normally immutable :)  Unfortunately many of these optimizations were semi-horrid hacks and introduced things like ModifiableKeyValues, so they all haven't made their way to apache.

In the end, after our optimizations, the real world workload Dhruba and I were working with was not all in-memory so the bottleneck in production became the random reads (so increasing the block cache hit ratio is the focus) rather than CPU contention or HLog throughput.

JG

From: Akash Ashok [mailto:[EMAIL PROTECTED]]
Sent: Sunday, October 23, 2011 2:57 AM
To: [EMAIL PROTECTED]
Subject: Re: SILT - nice keyvalue store paper

I was running some similar tests and came across a surprising finding. I compared reads and write on ConcurrentSkipListMap ( which the memstore uses) and synchronized TreeMap ( Which was literally treemap synchronized). Executed concurrent reads, writes and deletes on both of them.
Surprisingly synchronized treeMap performed better, though just slightly better, than ConcurrentSkipListMap which KeyValueSkipListSet uses.

Here are the output of a few runs

Sometimes the difference was considerable
Using HBaseMap it took 20438ms
Using TreeMap it took 11613ms
Time Difference:8825ms

And sometimes the difference was negligible
Using HBaseMap it took 13370ms
Using TreeMap it took 9482ms
Time Difference:3888ms

I've attaching the test  java file which I wrote to test it.
This might be a very minor differece but still surprising considering the fact that ConcurrentSkipListMap uses fancy 2 level indexes which they say improves the deletion performance.

And here are the details about the test run.
100 Threads each fetching 1,000,000 records
100 threads each adding 1,000,000 records.
100 threads each deletin 1,000,000 records
( Reads, Writes and deletes simultaneously )

Cheers,
Akash A
On Sun, Oct 23, 2011 at 3:25 AM, Stack <[EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>> wrote:
On Sat, Oct 22, 2011 at 2:41 PM, N Keywal <[EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>> wrote:
Yes, WAL is the long pole writing.  But MemStore has issues too;
Dhruba says cpu above.  Reading and writing it is also 'slow'.
St.Ack
NEW: Monitor These Apps!
elasticsearch, apache solr, apache hbase, hadoop, redis, casssandra, amazon cloudwatch, mysql, memcached, apache kafka, apache zookeeper, apache storm, ubuntu, centOS, red hat, debian, puppet labs, java, senseiDB