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From Ran Tavory <ran...@gmail.com>
Subject Re: performance tuning - where does the slowness come from?
Date Thu, 06 May 2010 17:56:08 GMT
Jonathan, I think it's the case of large values in the columns. The
problematic CF is a key-value store, so it has only one column per row,
however the value of that column can be large. It's a java serialized object
(uncompressed) which, may be 100s of bytes, maybe even a few megs. This CF
also suffers from zero cache hits since each time a read is for a unique
key.

I ran stress.py and I see much better results (reads are < 1ms) so I assume
my cluster is healthy, so I need to fix the app. Would 1meg bytes object
explain a 30ms (sometimes even more) read latency? The boxes aren't fancy,
not sure exactly what hardware we have there but it's "commodity"...

Thanks!

On Thu, May 6, 2010 at 5:22 PM, Jonathan Ellis <jbellis@gmail.com> wrote:

> columns, not CFs.
>
> put another way, how wide are the rows in the slow CF?
>
> On Wed, May 5, 2010 at 11:30 PM, Ran Tavory <rantav@gmail.com> wrote:
> > I have a few CFs but the one I'm seeing slowness in, which is the one
> with
> > plenty of cache misses has only one column per key.
> > Latency varies b/w 10m and 60ms but I'd say average is 30ms.
> >
> > On Thu, May 6, 2010 at 4:25 AM, Jonathan Ellis <jbellis@gmail.com>
> wrote:
> >>
> >> How many columns are in the rows you are reading from?
> >>
> >> 30ms is quite high, so I suspect you have relatively large rows, in
> >> which case decreasing the column index threshold may help.
>
> --
> Jonathan Ellis
> Project Chair, Apache Cassandra
> co-founder of Riptano, the source for professional Cassandra support
> http://riptano.com
>

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