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From Eran Chinthaka Withana <eran.chinth...@gmail.com>
Subject Re: Key cache hit rate issue
Date Thu, 16 Feb 2012 21:52:38 GMT
Hi Jonathan,

Thanks for the reply. Yes there is a possibility that the keys can be
distributed in multiple SSTables, but my data access patterns are such that
I always read/write the whole row. So I expect all the data to be in the
same SSTable (please correct me if I'm wrong).

For some reason 16637958 (the keys cached) has become a golden number and I
don't see key cache increasing beyond that. I also checked memory and I
have about 4GB left in JVM memory and didn't see any issues on logs.

Thanks,
Eran Chinthaka Withana


On Thu, Feb 16, 2012 at 1:20 PM, Jonathan Ellis <jbellis@gmail.com> wrote:

> So, you have roughly 1/6 of your (physical) row keys cached and about
> 1/4 cache hit rate, which doesn't sound unreasonable to me.  Remember,
> each logical key may be spread across multiple physical sstables --
> each (key, sstable) pair is one entry in the key cache.
>
> On Thu, Feb 16, 2012 at 1:48 PM, Eran Chinthaka Withana
> <eran.chinthaka@gmail.com> wrote:
> > Hi Aaron,
> >
> > Here it is.
> >
> > Keyspace: XXXX
> > Read Count: 1123637972
> > Read Latency: 5.757938114343114 ms.
> > Write Count: 128201833
> > Write Latency: 0.0682576607387509 ms.
> > Pending Tasks: 0
> > Column Family: YY
> > SSTable count: 18
> > Space used (live): 103318720685
> > Space used (total): 103318720685
> > Number of Keys (estimate): 92404992
> > Memtable Columns Count: 1425580
> > Memtable Data Size: 359655747
> > Memtable Switch Count: 2522
> > Read Count: 1123637972
> > Read Latency: 14.731 ms.
> > Write Count: 128201833
> > Write Latency: NaN ms.
> > Pending Tasks: 0
> > Bloom Filter False Postives: 1488
> > Bloom Filter False Ratio: 0.00000
> > Bloom Filter Space Used: 331522920
> > Key cache capacity: 16637958
> > Key cache size: 16637958
> > Key cache hit rate: 0.2708333333333333
> > Row cache: disabled
> > Compacted row minimum size: 51
> > Compacted row maximum size: 6866
> > Compacted row mean size: 2560
> >
> > Thanks,
> > Eran Chinthaka Withana
> >
> >
> >
> > On Thu, Feb 16, 2012 at 12:30 AM, aaron morton <aaron@thelastpickle.com>
> > wrote:
> >>
> >> Its in the order of 261 to 8000 and the ratio is 0.00. But i guess 8000
> is
> >> bit high. Is there a way to fix/improve it?
> >>
> >> Sorry I don't understand what you mean. But if the ratio is 0.0 all is
> >> good.
> >>
> >> Could you include the full output from cfstats for the CF you are
> looking
> >> at ?
> >>
> >> Cheers
> >>
> >> -----------------
> >> Aaron Morton
> >> Freelance Developer
> >> @aaronmorton
> >> http://www.thelastpickle.com
> >>
> >> On 15/02/2012, at 1:00 PM, Eran Chinthaka Withana wrote:
> >>
> >> Its in the order of 261 to 8000 and the ratio is 0.00. But i guess 8000
> is
> >> bit high. Is there a way to fix/improve it?
> >>
> >> Thanks,
> >> Eran Chinthaka Withana
> >>
> >>
> >> On Tue, Feb 14, 2012 at 3:42 PM, aaron morton <aaron@thelastpickle.com>
> >> wrote:
> >>>
> >>> Out of interest what does cfstats say about the bloom filter stats ? A
> >>> high false positive could lead to a low key cache hit rate.
> >>>
> >>> Also, is there a way to warm start the key cache, meaning pre-load the
> >>> amount of keys I set as keys_cached?
> >>>
> >>> See key_cache_save_period when creating the CF.
> >>>
> >>> Cheers
> >>>
> >>>
> >>> -----------------
> >>> Aaron Morton
> >>> Freelance Developer
> >>> @aaronmorton
> >>> http://www.thelastpickle.com
> >>>
> >>> On 15/02/2012, at 5:54 AM, Eran Chinthaka Withana wrote:
> >>>
> >>> Hi,
> >>>
> >>> I'm using Cassandra 1.0.7 and I've set the keys_cached to about 80%
> >>> (using the numerical values). This is visible in cfstats too. But I'm
> >>> getting less than 20% (or sometimes even 0%) key cache hit rate. Well,
> the
> >>> data access pattern is not the issue here as I know they are
> retrieving the
> >>> same row multiple times. I'm using hector client with dynamic load
> balancing
> >>> policy with consistency ONE for both reads and writes. Any ideas on
> how to
> >>> find the issue and fix this?
> >>>
> >>> Here is what I see on cfstats.
> >>>
> >>> Key cache capacity: 16637958
> >>> Key cache size: 16637958
> >>> Key cache hit rate: 0.045454545454545456
> >>>
> >>> Also, is there a way to warm start the key cache, meaning pre-load the
> >>> amount of keys I set as keys_cached?
> >>>
> >>> Thanks,
> >>> Eran
> >>>
> >>>
> >>
> >>
> >
>
>
>
> --
> Jonathan Ellis
> Project Chair, Apache Cassandra
> co-founder of DataStax, the source for professional Cassandra support
> http://www.datastax.com
>

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