Re: blk-mq: bitmap tag: performance degradation?

[Jens Axboe <axboe@kernel.dk>]

On 2014-06-05 08:01, Alexander Gordeev wrote:
> On Wed, Jun 04, 2014 at 08:18:42AM -0600, Jens Axboe wrote:
>> A null_blk test is the absolute best case for percpu_ida, since
>> there are enough tags and everything is localized. The above test is
>> more useful for testing blk-mq than any real world application of
>> the tagging.
>>
>> I've done considerable testing on both 2 and 4 socket (32 and 64
>> CPUs) and bitmap tagging is better in a much wider range of
>> applications. This includes even high tag depth devices like nvme,
>> and more normal ranges like mtip32xx and scsi-mq setups.
>
> Just for the record: bitmap tags on a 48 CPU box with NVMe device
> indeed shows almost the same performance/cache rate as the stock
> kernel.

Thanks for confirming. It's one of the dangers of null_blk, it's not 
always a very accurate simulation of what a real device will do. I think 
it's mostly a completion side thing, would be great with a small device 
that supported msi-x and could be used as an irq trigger :-)

-- 
Jens Axboe