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From "David Dabbs" <>
Subject RE: Performance fix in event mpm
Date Thu, 03 Feb 2011 00:02:22 GMT

Has anyone compared before/after performance when pounding a pre-patched
httpd (with ab or other load generator) and with the fdqueue mods? 
Or, for those more daring readers, observed improvements in a production
Before deploying a manually patched 2.2.x branch, we're probably going to
run some sort of load test. 
Having read the thread, I don't think we'd need to do anything other than
throw a lot of load at it, right?



-----Original Message-----
From: Niklas Edmundsson [] 
Sent: Friday, January 28, 2011 8:58 AM
Subject: Re: Performance fix in event mpm

On Fri, 28 Jan 2011, Jim Jagielski wrote:

> I was going to submit it as a backport, yes.

I have a strong feeling that this can explain the weird performance 
issues/behavior we've seen when hitting any bottleneck that results in 
requests being queued up.

Thanks for finding/fixing this :)

> On Jan 27, 2011, at 9:08 PM, David Dabbs wrote:
>> I see that the changes described below were applied to the trunk worker
>> event MPM code.
>> Would you consider applying it to the 2.2x branch? I will do so myself
>> test in my env.
>> Many thanks,
>> David Dabbs
>> -----Original Message-----
>> From: Jim Jagielski []
>> Sent: Thursday, January 27, 2011 12:43 PM
>> To:
>> Subject: Re: Performance fix in event mpm
>> On Jan 27, 2011, at 1:31 PM, Jim Jagielski wrote:
>>> On Jan 27, 2011, at 12:21 PM, Jim Van Fleet wrote:
>>>> I have been developing an application using apache 2.2 on linux 2.6.
>> test environment creates a very heavy workload and puts a strain on every
>> thing.
>>>> I would get good performance for a while and as the load ramped up,
>> performance would quickly get very bad.  Erratically, transactions would
>> finish quickly or take a very long time -- tcpdump analysis showed
>> millisecond or seconds between responses. Also, the recv queue got very
>> large.
>>>> I noticed that ap_queue_pop removes elements from the queue LIFO rather
>> than FIFO.  Also noticed that apr_queue_pop uses a different technique
>> is not too expensive and is fifo, so I changed ap_queue/pop/push to use
>> technique and the receive problems went away.
>>>> Please let me know if you think this change is appropriate and/or if
>> you'd like more data
>>> Hmmm.... Not sure why the fdqueue would be LIFO. But certainly
>>> the above ain't right for pop! :)
>> OK, looking over the history, it looks like the Q was changed from
>> FIFO to LIFO ~10years ago (worker)... The reasoning:
>>  This is a rather simple patch that may improve cache-hit performance
>>  under some conditions by changing the queue of available worker threads
>>  from FIFO to LIFO. It also adds a tiny reduction in the arithmetic that
>>  happens in the critical section, which will definately help if you have
>>  a lame compiler.
>> Seems to me that changing back to FIFO would make sense, esp
>> with trunk. We can profile the expense of the '% queue->bounds'
>> but it seems to me that if it was really bad, we'd have seen it
>> in apr and changed it there... after all, all we're doing
>> with that is keeping it in bounds and a comparison and subtraction
>> would do that just as well...

  Niklas Edmundsson, Admin @ {acc,hpc2n}      |
  I am Bashir on Borg: I'd be hostile to if my poop was cubed!

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