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From lars hofhansl <la...@apache.org>
Subject Re: Unscientific comparison of fully-cached zipfian reading
Date Wed, 22 May 2013 01:08:44 GMT
I just did a similar test using PE on a test cluster (16 DNs/RSs, 158 mappers).
I set it up such that the data does not fit into the aggregate block cache, but does fit into
the aggregate OS buffer cache, in my case that turned out to be 100m 1k rows.
Now I ran the SequentialRead and RandomRead tests.

In both cases I see no disk activity (since the data fits into the OS cache). The SequentialRead
run finishes in about 7mins, whereas the RandomRead run takes over 34mins.
This is with CDH4.2.1 and HBase 0.94.7 compiled against it and with SCR enabled.

The only difference is that in the SequentialRead case it is likely that the next Get can
still use the previously cached block, whereas in the RandomRead read almost every Get need
to fetch a block from the OS cache (as verified by the cache miss rate, which is roughly the
same as the request count per RegionServer). Except for enabling SCR all other settings are
close to the defaults.

I see 2000-4000 req/s/regionserver and the same number of cache missed per second and RegionServer
in the RandomRead, meaning each RegionServer brought in about 125-200mb/s from the OS cache,
which seems a tad low.

So this would imply that reading from the OS cache is almost 5x slower than reading from the
block cache. It would be interesting to explore the discrepancy.

-- Lars

 From: Jean-Daniel Cryans <jdcryans@apache.org>
To: "dev@hbase.apache.org" <dev@hbase.apache.org> 
Sent: Wednesday, April 24, 2013 6:01 PM
Subject: Unscientific comparison of fully-cached zipfian reading

Hey guys,

I did a little benchmarking to see what kind of numbers we get from the
block cache and the OS cache. Please see:


Hopefully it gives you some ballpark numbers for further discussion.

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