lkml.org 
[lkml]   [2006]   [Aug]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: RFC - how to balance Dirty+Writeback in the face of slow writeback.
From
Date
On Thu, 2006-08-17 at 13:59 +1000, Neil Brown wrote:
> 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.

Exactly how would a request limit help? All that boils down to is having
the VM monitor global_page_state(NR_FILE_DIRTY) versus monitoring
global_page_state(NR_FILE_DIRTY)+global_page_state(NR_WRITEBACK).

Cheers,
Trond

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2006-08-17 15:25    [from the cache]
©2003-2011 Jasper Spaans. Advertise on this site