> -----Original Message----- > From: Jens Axboe [mailto:axboe@xxxxxxxxx] > Sent: Tuesday, May 06, 2014 7:47 PM > To: Desai, Kashyap > Cc: linux-scsi@xxxxxxxxxxxxxxx > Subject: Re: How to get more sequential IO merged at elevator > > On 05/06/2014 04:06 AM, Desai, Kashyap wrote: > > I got some clue on what was going on while doing 4K sequential read using > fio. > > > > If I use ioengine in fio script as "libaio", I see " do_io_submit" call > plug/unplug the queue before submitting the IO. > > It means after every IO, we expect to send IO immediately to the next > layer. If at all there are any pending IO do merge.. but not due to plugging. > > > > This is what happens on my test. Every time IO comes from application > > with libaio engine, it send down to the elevator/io-scheduler because > queue was unplugged in do_io_submit(). Moment I reduce the queue depth > of the block device, merge start because of congestion at scsi mid layer. > > > > If I use, mmap engine, I see merged IO coming to the device driver > > because of plugging. I really don't know how it works, but gave a try > > and found merge happen because of plugging. ( I confirm using > > blktrace) > > > > Is there any ioengine in <fio> (or any other parameter setting), which can > use plugging mechanism of block layer to merge more IO other than mmap ? > > O_DIRECT IO is sync by nature, which is why it is sent off immediately instead > of held for potentially merging. mmap is not. You should be able to provoke > merging by submitting more than 1 IO at the time. See the iodepth_batch > settings for fio. Thanks Jens. I got your point about O_DIRECT IO. When I read <man> page of <fio> it mentioned default iodepth_batch and iodepth_low will be same as iodepth value.. but in my case I have to explicitly provide those parameters. (I explore this after you pointed out ). It looks like default value for iodepth_batch/low is not same as iodepth. Once I provide fio parameter "iodepth_batch=32" and "iodepth_low=32", I see IO in batch before plug and after unplug from application. I am able to get now max merged IO. Thanks for helping me on this. ~ Kashyap > > -- > Jens Axboe > -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
