Re: [RESEND] [PATCH] readahead:add blk_run_backing_dev

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

sorry for late responce.
I wonder why I and Wu don't contain Cc list in this thread.


> Hisashi Hifumi wrote:
> > At 09:36 09/06/01, Andrew Morton wrote:
> >> On Fri, 29 May 2009 14:35:55 +0900 Hisashi Hifumi 
> >> <hifumi.hisashi@oss.ntt.co.jp> wrote:
> >>
> >>> I added blk_run_backing_dev on page_cache_async_readahead
> >>> so readahead I/O is unpluged to improve throughput on 
> >>> especially RAID environment. 
> >> I skipped the last version of this because KOSAKI Motohiro
> >> <kosaki.motohiro@jp.fujitsu.com> said "Please attach blktrace analysis ;)".
> >>
> >> I'm not sure why he asked for that, but he's a smart chap and
> >> presumably had his reasons.
> >>
> >> If you think that such an analysis is unneeded, or isn't worth the time
> >> to generate then please tell us that.  But please don't just ignore the
> >> request!
> > 
> > Hi Andrew.
> > 
> > Sorry for this.
> > 
> > I did not ignore KOSAKI Motohiro's request.
> > I've got blktrace output for both with and without the patch, 
> > but I just did not clarify the reason for throuput improvement
> > from this result.
> > 
> > I do not notice any difference except around unplug behavior by dd.
> > Comments?
> 
> Pardon my ignorance on the global issues concerning the patch, but 
> specifically looking at the traces generated by blktrace leads one to 
> also note that the patched version may generate inefficiencies in other 
> places in the kernel by reducing the merging going on. In the unpatched 
> version it looks like (generally) that two incoming bio's are able to be 
> merged to generate a single I/O request. In the patched version - 
> because of the quicker unplug(?) - no such merging is going on. This 
> leads to more work lower in the stack (twice as many I/O operations 
> being managed), perhaps increased interrupts & handling &c. [This may be 
> acceptable if the goal is to decrease latencies on a per-bio basis...]
> 
> Do you have a place where the raw blktrace data can be retrieved for 
> more in-depth analysis?

I think your comment is really adequate. In another thread, Wu Fengguang pointed
out the same issue.
I and Wu also wait his analysis.

Thanks.