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From Huw Selley <huw.sel...@netdev.co.uk>
Subject Re: Read request throughput
Date Thu, 02 Dec 2010 11:29:38 GMT
Thanks for the response Adam :) 
Some updates below:

On 1 Dec 2010, at 13:30, Adam Kocoloski wrote:
<snip>
> 
> So the Erlang VM starts 16 schedulers by default, right?  Some people have reported improvements
in Erlang application performance with HyperThreading disabled, but I've not heard of any
CouchDB-specific tests of that option yet.

Yeah, that's right - 16:16 by default.

<snip>

> 
>> The database is pretty small (just under 100K docs) and I am querying a view that
includes some other docs (the request contains include_docs=true) and using jmeter on another
identical box to generate the traffic.
> 
> include_docs=true is definitely more work at read time than embedding the docs in the
view index.  I'm not sure  about your application design constraints, but given that your
database and index seem to fit entirely in RAM at the moment you could experiment with emitting
the doc in your map function instead ...
> 
>> The total amount of data returned from the request is 1467 bytes.
> 
> ... especially when the documents are this small.

Sure, but I would have expected that to only really help if the system was contending for
resources? I am using linked docs so not sure about emitting the entire doc in the view.

<snip>

>> 
>> 
>> Before coming here to question my findings I took a 3rd box (same spec) and built
couch from the tip of the 1.1.x branch (rev 1040477). After compiling couch and installing
it I found that it didn't start up (or log anything useful). After a bit of digging I figured
it's probably due to the age of the erlang version being used - I upgraded to OTP R14B and
rebuilt couch against it. This gave me a working install again.
> 
> Hmm, I've heard that we did something to break compatibility with 12B-5 recently.  We
should either fix it or bump the required version.  Thanks for the note.

COUCHDB-856?

> 
>> I got an immediate throughput increase to ~500 requests/s which was nice but the
data being collected via sadc still showed that the cpu was at most 20% utilised and the disk
controller was doing next to nothing (I assume the OS cache already has the data requested
so no trip to disk required?)
>> 
>> At this point I started to wonder if jmeter is unable to send in enough requests
to stress couch so I started up another jmeter instance on another box and had it also send
in requests to couch. What i noticed was that the total throughput didn't increase - it was
just split over both jmeter instances.
> 
> How many concurrent requests are submitted by each jmeter instance?

25.

<snip>

> 
> Do you know if the CPU load was spread across cores or concentrated on a single one?
 One thing Kenneth did not mention in that thread is that you can now bind Erlang schedulers
to specific cores.  By default the schedulers are unbound; maybe RHEL is doing a poor job
of distributing them.  You can bind them using the default strategy for your CPUs by starting
the VM with the "+sbt db" option.

It was using most of 2 cores. I had a go with "+sbt db" and it didn't perform as well as "-S
16:2".

WRT disabling HT - I need to take a trip to the datacentre to disable HT in the bios but I
tried disabling some cores with:

echo 0 > /sys/devices/system/node/nodeX/cpuX/online

Which should stop the kernel seeing the core - not as clean as disabling it in the bios but
should suffice. /proc/cpuinfo stopped showing the cores I removed so it looks like it worked.
Again I didn't see any improvement.

Cheers
Huw
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