cassandra-user mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Sylvain Lebresne <sylv...@datastax.com>
Subject Re: ParNew and caching
Date Fri, 18 Nov 2011 15:47:56 GMT
On Fri, Nov 18, 2011 at 4:23 PM, Mohit Anchlia <mohitanchlia@gmail.com> wrote:
> On Fri, Nov 18, 2011 at 6:39 AM, Sylvain Lebresne <sylvain@datastax.com> wrote:
>> On Fri, Nov 18, 2011 at 1:53 AM, Todd Burruss <bburruss@expedia.com> wrote:
>>> I'm using cassandra 1.0.  Been doing some testing on using cass's cache.
>>>  When I turn it on (using the CLI) I see ParNew jump from 3-4ms to
>>> 200-300ms.  This really screws with response times, which jump from ~25-30ms
>>> to 1300+ms.  I've increase new gen and that helps, but still this is
>>> suprising to me, especially since 1.0 defaults to the
>>> SerializingCacheProvider – off heap.
>>> The interesting tid bit is that I have wide rows.  70k+ columns per row, ~50
>>> bytes per column value.  The cache only must be about 400 rows to catch all
>>> the data per node and JMX is reporting 100% cache hits.  Nodetool ring
>>> reports < 2gb per node, my heap is 6gb and total RAM is 16gb.
>>> Thoughts?
>>
>> You're problem is the mix of wide rows and the serializing cache.
>> What happens with the serializing cache is that our data is stored
>> out of the heap. But that means that for each read to a row, we
>> 'deserialize' the row for the out-of-heap memory into the heap to
>> return it. The thing is, when we do that, we do the full row each
>> time. In other word, for each query we deserialize 70k+ columns
>> even if to return only one. I'm willing to bet this is what is killing
>> your response time. If you want to cache wide rows, I really
>> suggest you're using the ConcurrentLinkedHashCacheProvider
>> instead.
>
> What happens when using ConcurrentLinkedHashCache? What is the
> implementation like and why is it better?

With ConcurrentLinkedHashCache, the cache is in the heap. So there
is no deserialization/copy during gets, so having wide rows is not a
problem. Outside of the fact that if you're enabling cache on a column
family with wide rows, you have to keep in mind that we always keep
full rows in cache.

>
>>
>> I'll also note that this explain the ParNew times too. Deserializing
>> all those columns from off-heap creates lots of short-lived object,
>> and since you deserialize 70k+ on each query, that's quite some
>> pressure on the new gen. Note that the serializing cache is
>> actually minimizing the use of old gen, because that is the one
>> that is the one that can create huge GC pauses with big heap,
>> but it actually put more pressure on the new gen. This is by
>> design and because new gen is much less of a problem than
>> old gen.
>
> In this scenario would it help if Young generation space is increased?

That's a hard one to answer because GC tuning is a bit of a black
art, when testing and benchmarking is often key. Having a bigger
young generation means having young collection kicked less often
but on the other side it reduces the size for the old generation.
But again, I don't think the problem is really the GC here, at least not
primarily.

--
Sylvain

>
>>
>> --
>> Sylvain
>>
>

Mime
View raw message