linux-kernel - Re: [PATCH 11/11] DIO: optimize cache misses in the submission path

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Date:	Mon, 08 Aug 2011 15:38:03 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	hch@...radead.org, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 11/11] DIO: optimize cache misses in the submission path

Andi Kleen <andi@...stfloor.org> writes:

>> I bet we could default to using the smaller block size all the time, and
>> still be able to detect when we don't have to do the sub-block zeroing.
>> Maybe that would be a good follow-on patch.
>
> It doesn't really matter because it's out of the fast path now.
>
>> > +	/* 
>> > +	 * Avoid references to bdev if not absolutely needed to give
>> > +	 * the early prefetch in the caller enough time.
>> > +	 */
>> >  
>> > -	if (offset & blocksize_mask) {
>> > +	if (unlikely(offset & blocksize_mask)) {
>> 
>> You can't make this assumption.  Userspace controls what size/alignment
>> of blocks to send in.
>
> What assumption do you mean?

Sorry, I meant that we don't know whether the offset & blocksize_mask
check is unlikely.

Cheers,
Jeff
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