Re: [PATCH RFC 0/2] percpu_ida: Take into account CPU topology when stealing tags

[Jens Axboe]

On 05/01/2014 11:05 PM, Christoph Hellwig wrote:
> 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.

In practice it comes out to the same, it's not feasible to run a much
larger space and track on queue depth. So I don't think that changes the
conclusion for SCSI.


-- 
Jens Axboe

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