linux-kernel - Re: [PATCH] Changed blk trace msgs to directly use relay buffer

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Date:	Wed, 28 May 2008 09:22:25 -0400
From:	"Alan D. Brunelle" <Alan.Brunelle@...com>
To:	Jens Axboe <jens.axboe@...cle.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-btrace@...r.kernel.org
Subject: Re: [PATCH] Changed blk trace msgs to directly use relay buffer

Jens Axboe wrote:
> On Wed, May 28 2008, Alan D. Brunelle wrote:
>> Jens Axboe wrote:
>>> On Tue, May 27 2008, Alan D. Brunelle wrote:
>>>
>>>> From 43c8ea2b78f31d7ccd349384a9a2084e787aafc1 Mon Sep 17 00:00:00 2001
>>>> From: Alan D. Brunelle <alan.brunelle@...com>
>>>> Date: Tue, 27 May 2008 10:32:36 -0400
>>>> Subject: [PATCH] Changed blk trace msgs to directly use relay buffer
>>>>
>>>> Allows for SMP-usage without corruption, and removes an extra copy at
>>>> the expense of copying extra bytes. Reduced message size from 1024 to 128.
>>> Or, alternatively, something like the below. Then we don't
>>> unconditionally reserve and copy 128 bytes for each message, at the
>>> cost 128 bytes per-cpu per trace.
>> I looked into something like this, but thought the added complexity
>> wasn't worth it. Besides the extra per-cpu stuff, you also have an
>> extra memcopy involved - in my patch you print directly into the relay
>> buffer.  I figure that /if/ copying (128-msg_size) extra bytes is too
>> much, one could always shrink the 128 down further. [I would think 64
>> bytes is probably ok.]
>>
>> I'd bet that the reduced complexity, and skipping the extra memcopy
>> more than offsets having to copy a few extra bytes...
> 
> The complexity is the same imho, both versions are fairly trivial.
> I wasn't out to optimize this in a memory copy sense. To me the most
> precious resource is the data stream to the app, and 128 bytes
> is probably 6 times larger than the normal message would be. With
> the actual trace structure, we are down to about 3 times the byte
> size.
> 
> So it was just an idea, I don't care much either way. With 128 bytes,
> we could just put the buffer on the stack (and still do the copy to
> the relay buffer). The per-cpu buffers has the advantage that we
> could grow the size easily if we wanted to.
> 
> So, given everything, which do you prefer?
> 

Given your prioritizing of relay-copying over kernel-copying, I think
you're solution is better (and more flexible going forward).

Alan
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