Re: write-behind on streaming writes

[Vivek Goyal]

On Tue, Jun 05, 2012 at 01:23:02PM -0400, Vivek Goyal wrote:
> On Wed, May 30, 2012 at 11:21:29AM +0800, Fengguang Wu wrote:
> 
> [..]
> > (2) comes from the use of _WAIT_ flags in
> > 
> >         sync_file_range(..., SYNC_FILE_RANGE_WAIT_BEFORE|SYNC_FILE_RANGE_WRITE|SYNC_FILE_RANGE_WAIT_AFTER);
> > 
> > Each sync_file_range() syscall will submit 8MB write IO and wait for
> > completion. That means the async write IO queue constantly swing
> > between 0 and 8MB fillness at the frequency (100MBps / 8MB = 12.5ms).
> > So on every 12.5ms, the async IO queue runs empty, which gives any
> > pending read IO (from firefox etc.) a chance to be serviced. Nice
> > and sweet breaks!
> 
> I doubt that async IO queue is empty for 12.5ms. We wait for previous
> range to finish (index-1) and have already started the IO on next 8MB
> of pages. So effectively that should keep 8MB of async IO in
> queue (until and unless there are delays from user space side). So reason
> for latency improvement might be something else and not because async
> IO queue is empty for some time.

With sync_file_range() test, we can have 8MB of IO in flight. Without that
I think we can have more at times and that might be the reason for latency
improvement.

I see that CFQ has code to allow deeper NCQ depth if there is only a single
writer. So once a reader comes along it might find tons of async IO
already in flight. sync_file_range() will limit that in flight IO hence
the latency improvement. So if we have multiple dd doing sync_file_range()
then probably this latency improvement should go away.

I will run some tests to verify if my understanding about deeper queue
depths in case of single writer is correct or not.

Thanks
Vivek
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