On Fri, Nov 18, 2011 at 6:39 AM, Sylvain Lebresne <sylv...@datastax.com> wrote: > On Fri, Nov 18, 2011 at 1:53 AM, Todd Burruss <bburr...@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? > > 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? > > -- > Sylvain >
