Re: [GIT PULL] Ext3 latency fixes

[Theodore Tso]

On Fri, Apr 03, 2009 at 11:24:50AM -0700, Linus Torvalds wrote:
> But at the same time, I now suspect that we could actually have solved 
> this problem more easily by just doing things the other way around: make 
> the default "WRITE" be the high-priority one (to match "READ"), and then 
> just explicitly marking the data writes with "WRITE_ASYNC".
> 
> Why? Because I think that with all the writes sprinkled around in random 
> places, it's probably _easier_ to find the bulk writes that cause the 
> biggest issues, and just fix _those_ to be WRITE_ASYNC. They may be bulk, 
> they may be the common case, but they also tend to be the case where we 
> write with generic routines (eg the whole "do_writepages()" thing).
> 
> So the VFS layer tends to already do much of the bulk writeout, and maybe 
> we would have been better off just changing those to ASYNC and leaving any 
> more specialized cases as the SYNC case? That would have avoided a lot of 
> this effort at the filesystem level. We'd just assume that the default 
> filesystem-specific writes tend to all be SYNC.

Well, most filesystem-specific writes tend all to be ASYNC; it's only
those related to commits and fsync() which are SYNC.  Ext3 is unusual
in that data=ordered and the physical-block journalling design of the
jbd layer means that we actually have a much larger number of blocks
that need to be written out synchronously than most other filesystems.
But even so, the number of callsites that I needed to change weren't
that large; in fact, over half of them weren't in the filesystem at
all, but in the page writeback code, since fsync() and data=ordered
both need to wait for the inodes's pages to be flushed out to disk,
and that's all done in common code.  The other 40% was in the jbd's
commit code, while we are writing out the journal buffers.

I suspect the more important thing to address is the fact that
WRITE_SYNC unplugs the block device queue, and we would be better off
separating marking a particular I/O as "a user is waiting for this"
from "unplug the device queue now".  That will hopefully improve
things even more.

					- Ted
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