linux-kernel - Re: [PATCH 03/13] scsi: unify allocation of scsi command and sense buffer

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Date:	Tue, 26 May 2009 19:28:14 +0300
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
CC:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	jens.axboe@...cle.com, rdreier@...co.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	chris.mason@...cle.com, david@...morbit.com, hch@...radead.org,
	akpm@...ux-foundation.org, jack@...e.cz,
	yanmin_zhang@...ux.intel.com, linux-scsi@...r.kernel.org
Subject: Re: [PATCH 03/13] scsi: unify allocation of scsi command and sense
 buffer

On 05/26/2009 07:12 PM, Boaz Harrosh wrote:
> On 05/26/2009 05:47 PM, James Bottomley wrote:
>> There are several other approaches:
>>
>>      1. Keep the sense buffer packed in the command but disallow DMA to
>>         it, which fixes all the alignment problems.  Then we supply a
>>         set of rotating DMA buffers to drivers which need to do the DMA
>>         (which isn't the majority).
> 
> This one is not possible because it is scsi-ml in majority of cases that
> does the DMA request through scsi_eh_prep_cmnd() and a regular read.
> The drivers don't even know anything about it.
> 

I retract that no, yes and scsi-ml is one more possible client of
the "rotating DMA buffers"

>>      2. Sense is a comparative rarity, so us a more compact pooling
>>         scheme and discard sense for reuse as soon as we know it's not
>>         used (as in at softirq time when there's no sense collected).
>>
> 
> This is the way to go for sure. And only on ARCHs with none-coherent-cache
> all the good ARCHs can just use embedded sense just fine.
> 
>> I'd need a little more clarity on the actual size of the problem before
>> making any choices.
>>
>> The other thing to bear in mind is that two allocations of M and N might
>> be more costly than a single allocation of N+M; however, an allocation
>> of M+N+extra can end up more costly if the extra causes more page
>> reclaim before we get an actual command.
>>
>> James
>>
> Boaz
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