linux-kernel - Re: Linux Kernel Markers - performance characterization with large IO load on large-ish system

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Date:	Tue, 02 Oct 2007 13:51:22 -0400
From:	"Alan D. Brunelle" <Alan.Brunelle@...com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Jens Axboe <jens.axboe@...cle.com>, linux-kernel@...r.kernel.org,
	btrace <linux-btrace@...r.kernel.org>
Subject: Re: Linux Kernel Markers - performance characterization with large
 IO load on large-ish system

Mathieu Desnoyers wrote:
>> remember that we have seen and discussed something like this before,
>> it's still a puzzle to me...
>>
>>     
> I do wonder about that performance _increase_ with blktrace enabled. I
>
> Interesting question indeed.
>
> In those tests, when blktrace is running, are the relay buffers only
> written to or they are also read ?
>   

blktrace (the utility) was running too - so the relay buffere /were/
being read and stored out to disk elsewhere.

> Running the tests without consuming the buffers (in overwrite mode)
> would tell us more about the nature of the disturbance causing the
> performance increase.
>   

I'd have to write a utility to enable the traces, but then not read. Let
me think about that.

> Also, a kernel trace could help us understand more thoroughly what is
> happening there.. is it caused by the scheduler ? memory allocation ?
> data cache alignment ?
>   

Yep - when I get some time, I'll look into that. [Clearly not a gating
issue for marker support...]

> I would suggest that you try aligning the block layer data structures
> accessed by blktrace on L2 cacheline size and compare the results (when
> blktrace is disabled).
>   

Again, when I get some time! :-)

Alan
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