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Re: [dm-devel] [PATCH v3 14/16] Gut bio_add_page()


On Mon, May 28, 2012 at 05:27:33PM -0400, Mikulas Patocka wrote:
> > They're split and made in-flight together.
> I was talking about old ATA disk (without command queueing). So the 
> requests are not sent together. USB 2 may be a similar case, it has 
> limited transfer size and it doesn't have command queueing too.

I meant in the block layer.  For consecutive commands, queueing
doesn't really matter.

> > Disk will most likely seek to the sector read all of them into buffer
> > at once and then serve the two consecutive commands back-to-back
> > without much inter-command delay.
> Without command queueing, the disk will serve the first request, then 
> receive the second request, and then serve the second request (hopefully 
> the data would be already prefetched after the first request).
> The point is that while the disk is processing the second request, the CPU 
> can already process data from the first request.

Those are transfer latencies - multiple orders of magnitude shorter
than IO latencies.  It would be surprising if they actually are
noticeable with any kind of disk bound workload.

> > Isn't it more like you shouldn't be sending read requested by user and
> > read ahead in the same bio?
> If the user calls read with 512 bytes, you would send bio for just one 
> sector. That's too small and you'd get worse performance because of higher 
> command overhead. You need to send larger bios.

All modern FSes are page granular, so the granularity would be
per-page.  Also, RAHEAD is treated differently in terms of
error-handling.  Do filesystems implement their own rahead
(independent from the common logic in vfs layer) on their own?

> AHCI can interrupt after partial transfer (so for example you can send a 
> command to read 1M, but signal interrupt after the first 4k was 
> transferred), but no one really wrote code that could use this feature. It 
> is questionable if this would improve performance because it would double 
> interrupt load.

The feature is pointless for disks anyway.  Think about the scales of
latencies of different phases of command processing.  The difference
is multiple orders of magnitude.

> > If exposing segmenting limit upwards is a must (I'm kinda skeptical),
> > let's have proper hints (or dynamic hinting interface) instead.
> With this patchset, you don't have to expose all the limits. You can 
> expose just a few most useful limits to avoid bio split in the cases 
> described above.

Yeah, if that actually helps, sure.  From what I read, dm is already
(ab)using merge_bvec_fn() like that anyway.



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