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

[Jens Axboe <axboe@kernel.dk>]

On 2014-06-04 04:35, Alexander Gordeev wrote:
> Hi Jens, et al
>
> With new bitmap tags I am observing performance degradation on 'null_blk'
> device with 512 queue depth. This is 'fio' config used:
>
> [global]
> bs=4k
> size=16g
>
> [nullb]
> filename=/dev/nullb0
> direct=1
> rw=randread
> numjobs=8
>
>
> I tried machines with 16 and 48 CPUs and it seems the more
> CPUs we have the worse the result. Here is 48 CPUs one:
>
> 3.15.0-rc4+
>
>     READ: io=131072MB, aggrb=3128.7MB/s, minb=400391KB/s, maxb=407204KB/s,
> mint=41201msec, maxt=41902msec
>
>     548,549,235,428 cycles:k
>       3,759,335,303 L1-dcache-load-misses
>         419,021,008 cache-misses:k
>
>        39.659121371 seconds time elapsed
>
> 3.15.0-rc1.for-3.16-blk-mq-tagging+
>
>     READ: io=131072MB, aggrb=1951.8MB/s, minb=249824KB/s, maxb=255851KB/s,
> mint=65574msec, maxt=67156msec
>
>   1,063,669,976,651 cycles:k
>       4,572,746,591 L1-dcache-load-misses
>       1,127,037,813 cache-misses:k
>
>        69.446112553 seconds time elapsed

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.

-- 
Jens Axboe