Re: [RFC PATCH] ext4: fix 50% disk write performance regression

[Ted Ts'o <tytso@mit.edu>]

On Mon, Aug 30, 2010 at 04:49:58PM -0400, Bill Fink wrote:
> > Thanks for reporting it.  I'm going to have to take a closer look at
> > why this makes a difference.  I'm going to guess though that what's
> > going on is that we're posting writes in such a way that they're no
> > longer aligned or ending at the end of a RAID5 stripe, causing a
> > read-modify-write pass.  That would easily explain the write
> > performance regression.
> 
> I'm not sure I understand.  How could calling or not calling
> ext4_num_dirty_pages() (unpatched versus patched 2.6.35 kernel)
> affect the write alignment?

Suppose you have 8 disks, with stripe size of 16k.  Assuming that
you're only using one parity disk (i.e., RAID 5) and no spare disks,
that means the optimal I/O size is 7*16k == 112k.  If we do a write
which is smaller than 112k, or which is not a multiple of 112k, then
the RAID subsystem will need to do a read-modify-write to update the
parity disk.  Furthermore, the write had better be aligned on an 112k
byte boundary.  The block allocator will guarantee that block #0 is
aligned on a 112k block, but writes have to also be right size in
order to avoid the read-modify-write.

If we end up doing very small writes, then it can end up being quite
disatrous for write performance.

> I was wondering if the locking being done in ext4_num_dirty_pages()
> could somehow be affecting the performance.  I did notice from top
> that in the patched 2.6.35 kernel, the I/O wait time was generally
> in the 60-65% range, while in the unpatched 2.6.35 kernel, it was
> at a higher 75-80% range.  However, I don't know if that's just a
> result of the lower performance, or a possible clue to its cause.

I/O wait time would tend to imply that the raid controller is taking
longer to do the write updates, which would tend to confirm that we're
doing more read-modify-write cycles.  If we were hitting spinlock
contention, this would show up as more system CPU time consumed.

	    	       	       	       - Ted