blk-mq: bitmap tag: performance degradation?

[Alexander Gordeev]

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

Thanks!

-- 
Regards,
Alexander Gordeev
agordeev@xxxxxxxxxx
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