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

[Mikulas Patocka]

On Tue, 29 May 2012, Tejun Heo wrote:

> Hello,
> 
> On Mon, May 28, 2012 at 12:07:14PM -0400, Mikulas Patocka wrote:
> > With accurately sized bios, you send one bio for 256 sectors (it is sent 
> > immediatelly to the disk) and a second bio for another 256 sectors (it is 
> > put to the block device queue). The first bio finishes, pages are marked 
> > as uptodate, the second bio is sent to the disk. While the disk is 
> 
> 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.

> > processing the second bio, the kernel already knows that the first 256 
> > sectors are finished - so it copies the data to userspace and lets the 
> > userspace process them - while the disk is processing the second bio. So, 
> > disk transfer and data processing are overlapped.
> >
> > Now, with your patch, you send just one 512-sector bio. The bio is split 
> > to two bios, the first one is sent to the disk and you wait. The disk 
> > finishes the first bio, you send the second bio to the disk and wait. The 
> > disk finishes the second bio. You complete the master bio, mark all 512 
> > sectors as uptodate in the pagecache, start copying data to the userspace 
> > and processing them. Disk transfer and data processing are not overlapped.
> 
> 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.

> > accurately-sized bios (that don't span stripe boundaries), each bio waits 
> > just for one disk to seek to the requested position. If you send oversized 
> > bio that spans several stripes, that bio will wait until all the disks 
> > seek to the requested position.
> > 
> > In general, you can send oversized bios if the user is waiting for all the 
> > data requested (for example O_DIRECT read or write). You shouldn't send 
> > oversized bios if the user is waiting just for a small part of data and 
> > the kernel is doing readahead - in this case, oversized bio will result in 
> > additional delay.
> 
> 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.

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.

> > I think bio_add_page should be simplified in such a way that in the most 
> > common cases it doesn't create oversized bio, but it can create oversized 
> > bios in uncommon cases. We could retain a limit on a maximum number of 
> > sectors (this limit is most commonly hit on disks), put a stripe boundary 
> > to queue_limits (the stripe boundary limit is most commonly hit on raid), 
> > ignore the rest of the limits in bio_add_page and remove merge_bvec.
> 
> 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.

> Thanks.
> 
> -- 
> tejun

Mikulas
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