Re: RFC - how to balance Dirty+Writeback in the face of slow writeback.

[Neil Brown <neilb@suse.de>]

On Tuesday August 15, akpm@osdl.org wrote:
> > When Dirty hits 0 (and Writeback is theoretically 80% of RAM)
> > balance_dirty_pages will no longer be able to flush the full
> > 'write_chunk' (1.5 times number of recent dirtied pages) and so will
> > spin in a loop calling blk_congestion_wait(WRITE, HZ/10), so it isn't
> > a busy loop, but it won't progress.
> 
> This assumes that the queues are unbounded.  They're not - they're limited
> to 128 requests, which is 60MB or so.

Ahhh... so the limit on the requests-per-queue is an important part of
write-throttling behaviour.  I didn't know that, thanks.

fs/nfs doesn't seem to impose a limit.  It will just allocate as many
as you ask for until you start running out of memory.  I've seen 60%
of memory (10 out of 16Gig) in writeback for NFS.

Maybe I should look there to address my current issue, though imposing
a system-wide writeback limit seems safer.

> 
> Per queue.  The scenario you identify can happen if it's spread across
> multiple disks simultaneously.
> 
> CFQ used to have 1024 requests and we did have problems with excessive
> numbers of writeback pages.  I fixed that in 2.6.early, but that seems to
> have got lost as well.
> 

What would you say constitutes "excessive"?  Is there any sense in
which some absolute number is excessive (as it takes too long to scan
some list) or is it just a percent-of-memory thing?

> 
> Something like that - it'll be relatively simple.

Unfortunately I think it is also relatively simple to get it badly
wrong:-)  Make one workload fast, and another slower.

But thanks, you've been very helpful (as usual).  I'll ponder it a bit
longer and see what turns up.

NeilBrown
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