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From Mohit Anchlia <>
Subject Re: ParNew and caching
Date Fri, 18 Nov 2011 22:40:41 GMT
On Fri, Nov 18, 2011 at 1:46 PM, Todd Burruss <> wrote:
> Ok, I figured something like that.  Switching to
> ConcurrentLinkedHashCacheProvider I see it is a lot better, but still
> instead of the 25-30ms response times I enjoyed with no caching, I'm
> seeing 500ms at 100% hit rate on the cache.  No old gen pressure at all,
> just ParNew crazy.

Are you saying that when you had off heap you saw better performance
of 25-30 ms? And now it's 500ms to get 50 columns? What kind of load
are you generating? What results do you see if you disabled row cache
and just leave key cache on?

There are lot of factors to consider so having more stats would be helpful.

Please paste cfhistograms. Have you tried monitoring tpstats and netstats?

What's your CL and RF?
> More info on my use case is that I am picking 50 columns from the 70k.
> Since the whole row is in the cache, and no copying from off-heap nor disk
> buffers, seems like it should be faster than non-cache mode.
> More thoughts :)
> On 11/18/11 6:39 AM, "Sylvain Lebresne" <> wrote:
>>On Fri, Nov 18, 2011 at 1:53 AM, Todd Burruss <>
>>> 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
>>> 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
>>> 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
>>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.

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