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Date: Thu, 1 May 2014 22:05:23 -0700
From: Christoph Hellwig <hch@infradead.org>
To: Jens Axboe <axboe@kernel.dk>
Cc: Kent Overstreet <kmo@daterainc.com>, Ming Lei <tom.leiming@gmail.com>,
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Subject: Re: [PATCH RFC 0/2] percpu_ida: Take into account CPU topology when
 stealing tags
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On Thu, May 01, 2014 at 08:19:39PM -0600, Jens Axboe wrote:
> I've taken the consequence of this and implemented another tagging
> scheme that blk-mq will use if it deems that percpu_ida isn't going
> to be effective for the device being initialized. But I really hate
> to have both of them in there. Unfortunately I have no devices
> available that have a tag space that will justify using percu_ida,
> so comparisons are a bit hard at the moment. NVMe should change
> that, though, so decision will have to be deferred until that is
> tested.

At least for SCSI devices _tag space_ is plenty, it's just the we
artifically limit our tag space to the queue depth to avoid having to
track that one separately.  In addition we also preallocaste a request
for each tag, so even if we would track the queue depth separately
we would waste a lot of memory.