Re: [PATCH] avoid too many boundaries in DIO

[David Chinner]

On Thu, Nov 09, 2006 at 08:48:54PM -0500, Chris Mason wrote:
> Dave Chinner found a 10% performance regression with ext3 when using DIO
> to fill holes instead of buffered IO.  On large IOs, the ext3 get_block
> routine will send more than a page worth of blocks back to DIO via a
> single buffer_head with a large b_size value.
> 
> The DIO code iterates through this massive block and tests for a
> boundary buffer over and over again.  For every block size unit spanned
> by the big map_bh, the boundary bit is tested and a bio may be forced
> down to the block layer.
> 
> There are two potential fixes, one is to ignore the boundary bit on
> large regions returned by the FS.  DIO can't tell which part of the big
> region was a boundary, and so it may not be a good idea to trust the
> hint.
> 
> This patch just clears the boundary bit after using it once.  It is 10%
> faster for a streaming DIO write w/blocksize of 512k on my sata drive.

8p altix, 8GB RAM, 64 FC disks, >2.5GiB/s sustainable raw throughput.
dm stripe, outer 1GB of each disk for 64GB volume. Chunk size 128k.
Single thread Direct I/O, I/O size of 512MiB, sequential file extend.

# mkfs.ext3 -E stride-size=32 /dev/mapper/testvol
# mkfs.xfs -f -d sunit=256,swidth=16384 /dev/mapper/testvol

ext3 mounted data=ordered; data=writeback results are the same
for direct I/O.

2.6.19-rc3-pl is 2.6.19-rc3 + the direct I/o placeholder patches.
2.6.19-rc3-pl-bd is 2.6.19-rc3-pl plus the boundary patch.

Kernel            fs    throughput      I/Os/s          sys+intr
-----------       ----  ----------      -------         ------
2.6.19-rc3        ext3   660MiB/s       ~36,000         70+60
2.6.19-rc3-pl     ext3   600MiB/s       ~36,000         70+60
2.6.19-rc3-pl-bd  ext3   715MiB/s       ~16,000         45+35
2.6.19-rc3        xfs   2.28GiB/s       ~18,000         65+65
2.6.19-rc3-pl     xfs   2.24GiB/s       ~18,000         65+65
2.6.19-rc3-pl-bd  xfs   2.26GiB/s       ~18,000         65+65

Hole filling with direct I/O shows equivalent results.

The boundary patch doubles the average I/O size of ext3 and
substantially reduces CPU usage for direct I/O. Nice one, Chris.

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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