Messages in this thread | | | Date | Tue, 10 Nov 2009 04:01:47 -0800 (PST) | From | Martin Knoblauch <> | Subject | Re: Likley stupid question on "throttle_vm_writeout" |
| |
----- Original Message ----
> From: Wu Fengguang <fengguang.wu@intel.com> > To: Peter Zijlstra <peterz@infradead.org> > Cc: Martin Knoblauch <spamtrap@knobisoft.de>; linux-kernel@vger.kernel.org > Sent: Tue, November 10, 2009 3:08:58 AM > Subject: Re: Likley stupid question on "throttle_vm_writeout" > > On Mon, Nov 09, 2009 at 04:26:33PM +0100, Peter Zijlstra wrote: > > On Mon, 2009-11-09 at 07:15 -0800, Martin Knoblauch wrote: > > > Hi, (please CC me on replies) > > > > > > I have a likely stupid question on the function "throttle_vm_writeout". > Looking at the code I find: > > > > > > if (global_page_state(NR_UNSTABLE_NFS) + > > > global_page_state(NR_WRITEBACK) <= dirty_thresh) > > > break; > > > congestion_wait(WRITE, HZ/10); > > > > > > Shouldn't the NR_FILE_DIRTY pages be considered as well? > > > > Ha, you just trod onto a piece of ugly I'd totally forgotten about ;-) > > > > The intent of throttle_vm_writeout() is to limit the total pages in > > writeout and to wait for them to go-away. > > Like this: > > vmscan fast => large NR_WRITEBACK => throttle vmscan based on it > > > Everybody hates the function, nobody managed to actually come up with > > anything better. > > btw, here is another reason to limit NR_WRITEBACK: I saw many > throttle_vm_writeout() waits if there is no wait queue to limit > NR_WRITEBACK (eg. NFS). In that case the (steadily) big NR_WRITEBACK > is _not_ caused by fast vmscan.. >
That is exactely what made me look again into the code. My observation is that when doing something like:
dd if=/dev/zero of=fast-local-disk bs=1M count=15000
most of the "dirty" pages are in NR_FILE_DIRTY with some relatively small amount (10% or so) in NR_WRITEBACK. If I do:
dd if=/dev/zero of=some-nfs-mount bs=1M count=15000
NR_WRITEBACK almost immediatelly goes up to dirty_ratio, with NR_UNSTABLE_NFS small. Over time NR_UNSTABLE_NFS grows, but is always lower than NR_WRITEBACK (maybe 40/60).
But don't ask what happens if I do both in parallel.... The local IO really slows to a crawl and sometimes the system just becomes very unresponsive. Have we heard that before? :-)
Somehow I have the impression that NFS writeout is able to absolutely dominate the dirty pages to an extent that the system is unusable.
Cheers Martin
| |