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From <Milind.Bhandar...@emc.com>
Subject Re: SILT - nice keyvalue store paper
Date Mon, 24 Oct 2011 17:29:04 GMT
A 3 year old article on DW:
http://www.ibm.com/developerworks/java/library/j-benchmark1/index.html is
very useful for benchmarking java code.

- milind

Milind Bhandarkar
Greenplum Labs, EMC
(Disclaimer: Opinions expressed in this email are those of the author, and
do not necessarily represent the views of any organization, past or
present, the author might be affiliated with.)

On 10/23/11 5:05 PM, "Jonathan Gray" <jgray@fb.com> wrote:

>Oh, and when running these experiments, you should look at the impact at
>which order they are run in, whether you run them multiple times per JVM
>instance, etc.  Basically, you need to be cognizant of the HotSpot
>optimizations the JVM is doing at runtime.
>> -----Original Message-----
>> From: Jonathan Gray [mailto:jgray@fb.com]
>> Sent: Sunday, October 23, 2011 4:20 PM
>> To: dev@hbase.apache.org
>> Subject: 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
>> contention at 1000 threads will kill you and that's when CSLM should do
>> better.  (1000 handler threads is approx. what I run with on RS in
>> Though I am a bit surprised that at 100 threads the TreeMap was
>> 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
>> impractically large MemStores and everything still being quite fast.
>> Over in Cassandra, I believe they have a two-level CSLM with the first
>> 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
>> they have one shard/region per node and we generally have several
>> 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
>> batch, 4ms per sync, so 62.5k req/sec.  (62.5k * 100 bytes/req =
>> 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
>> 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
>> 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
>> and working from that instead of starting at the root node, using an
>> and taking advantage of the available remove() semantics, or simply just
>> mutating things that are normally immutable :)  Unfortunately many of
>> 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
>> 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:thehellmaker@gmail.com]
>> Sent: Sunday, October 23, 2011 2:57 AM
>> To: dev@hbase.apache.org
>> 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
>> <stack@duboce.net<mailto:stack@duboce.net>> wrote:
>> On Sat, Oct 22, 2011 at 2:41 PM, N Keywal
>> <nkeywal@gmail.com<mailto:nkeywal@gmail.com>> wrote:
>> > I would think that the bottleneck for insert is the wal part?
>> > It would be possible to do a kind of memory list preparation during
>> > the wal insertion, and if the wal insertion is confirmed, do the
>> > insertion in the memory list. But it's strange to have the insertion
>> memory important vs.
>> > the insertion on disk...
>> >
>> Yes, WAL is the long pole writing.  But MemStore has issues too; Dhruba
>> cpu above.  Reading and writing it is also 'slow'.
>> St.Ack

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