On 8/28/14, Maxim Patlasov <mpatlasov@xxxxxxxxxxxxx> wrote: > On 08/21/2014 09:44 AM, Ming Lei wrote: >> On Wed, Aug 20, 2014 at 4:50 AM, Jens Axboe <axboe@xxxxxxxxx> wrote: >> >>> >>> Reworked a bit more: >>> >>> http://git.kernel.dk/?p=linux-block.git;a=commit;h=a323185a761b9a54dc340d383695b4205ea258b6 >> One big problem of the commit is that it is basically a serialized >> workqueue >> because of single &hctx->run_work, and per-req work_struct has to be >> used for concurrent implementation. So looks the approach isn't flexible >> enough compared with doing that in driver, or any idea about how to fix >> that? >> > > I'm interested what's the price of handling requests in a separate > thread at large. I used the following fio script: > > [global] > direct=1 > bsrange=512-512 > timeout=10 > numjobs=1 > ioengine=sync > > filename=/dev/loop0 # or /dev/nullb0 > > [f1] > rw=randwrite > > to compare the performance of: > > 1) /dev/loop0 of 3.17.0-rc1 with Ming's patches applied -- 11K iops If you enable BLK_MQ_F_WQ_CONTEXT, it isn't strange to see this result since blk-mq implements a serialized workqueue. > 2) the same as above, but call loop_queue_work() directly from > loop_queue_rq() -- 270K iops > 3) /dev/nullb0 of 3.17.0-rc1 -- 380K iops In my recent investigation and discussion with Jens, using workqueue may introduce some regression for cases like loop over null_blk, tmpfs. And 270K vs. 380K is a bit similar with my result, and it was observed that context switch is increased by more than 50% with introducing workqueue. I will post V3 which will use previous kthread, with blk-mq & kernel aio, which should make full use of blk-mq and kernel aio, and won't introduce regression for cases like above. > Taking into account so big difference (11K vs. 270K), would it be worthy > to implement pure non-blocking version of aio_kernel_submit() returning > error if blocking needed? Then loop driver (or any other in-kernel user) The kernel aio submit is very similar with user space's implementation, except for block plug&unplug usage in user space aio submit path. If it is blocked in aio_kernel_submit(), you should observe similar thing with io_submit() too. Thanks, -- Ming Lei -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
