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

["Alan D. Brunelle" <Alan.Brunelle@hp.com>]

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@hp.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...

Alan