CONFIG_SCSI_MQ_DEFAULT cause slow direct write speed to usb stick

[Andrej Gelenberg]

Hi,

i had found a problem with CONFIG_SCSI_MQ_DEFAULT config option. If it
activated, then the write speeds to the /dev/sd* of an usb stick drops
dramatically: it's only about 250 kb/CONFIG_SCSI_MQ_DEFAULTs, but
should be about 7 Mb/s. Git bisect also points to commit
24c20f10583647e30afe87b6f6d5e14bc7b1cbc6 'scsi: add a
CONFIG_SCSI_MQ_DEFAULT option' (i always set CONFIG_SCSI_MQ_DEFAULT to
y, because it sounded interesting). Same problem is also in 3.19
present. Write speeds to a filesystem on that USB-Stick were not so
bad, but as i tried to dd in Live-DVD Ubuntu image it was painfully
slow. After i disabled CONFIG_SCSI_MQ_DEFAULT write speeds are back to
normal.

I created bug report in bugzilla:
https://bugzilla.kernel.org/show_bug.cgi?id=92571

Regards,
Andrej
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