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HBase >> mail # dev >> Unscientific comparison of fully-cached zipfian reading


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Jean-Daniel Cryans 2013-04-25, 01:01
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Jean-Daniel Cryans 2013-04-25, 22:24
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lars hofhansl 2013-05-22, 01:08
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Jean-Daniel Cryans 2013-05-22, 17:23
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Re: Unscientific comparison of fully-cached zipfian reading
The 2000-4000 was just glancing at the HMaster page every now and then.
The main point I was trying to make is that the only difference is the number of block cache misses (which is low in the SequentialRead case and very high in the RandomRead case), and the number of cache misses is almost the same as the number of a requests.

(The cache misses are traced via OpenTSDB).

I'll repeat my test with a single region server only. Was your test in a cluster or with a single region server?
-- Lars

________________________________
 From: Jean-Daniel Cryans <[EMAIL PROTECTED]>
To: "[EMAIL PROTECTED]" <[EMAIL PROTECTED]>; lars hofhansl <[EMAIL PROTECTED]>
Sent: Wednesday, May 22, 2013 10:23 AM
Subject: Re: Unscientific comparison of fully-cached zipfian reading
 

On Tue, May 21, 2013 at 6:08 PM, lars hofhansl <[EMAIL PROTECTED]> wrote:
> 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.

That's a lot of variance. In my test the latencies I wrote there were
stable around those numbers. So we have a different way of measuring?

>
>
> 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 <[EMAIL PROTECTED]>
> To: "[EMAIL PROTECTED]" <[EMAIL PROTECTED]>
> 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:
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> https://docs.google.com/spreadsheet/pub?key=0Ao87IrzZJSaydENaem5USWg4TlRKcHl0dEtTS2NBOUE&output=html
>
> Hopefully it gives you some ballpark numbers for further discussion.
>
> J-D
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Jean-Daniel Cryans 2013-05-23, 17:00