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

Neil Brown
2006-08-17T03:59:47Z

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