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SubjectRe: [PATCH] mm: use global_dirty_limit in throttle_vm_writeout()
Hi Wu,

Sorry for late response.
I have a question.

On Fri, Mar 02, 2012 at 02:14:51PM +0800, Fengguang Wu wrote:
> When starting a memory hog task, a desktop box w/o swap is found to go
> unresponsive for a long time. It's solely caused by lots of congestion
> waits in throttle_vm_writeout():
>
> gnome-system-mo-4201 553.073384: congestion_wait: throttle_vm_writeout+0x70/0x7f shrink_mem_cgroup_zone+0x48f/0x4a1
> gnome-system-mo-4201 553.073386: writeback_congestion_wait: usec_timeout=100000 usec_delayed=100000
> gtali-4237 553.080377: congestion_wait: throttle_vm_writeout+0x70/0x7f shrink_mem_cgroup_zone+0x48f/0x4a1
> gtali-4237 553.080378: writeback_congestion_wait: usec_timeout=100000 usec_delayed=100000
> Xorg-3483 553.103375: congestion_wait: throttle_vm_writeout+0x70/0x7f shrink_mem_cgroup_zone+0x48f/0x4a1
> Xorg-3483 553.103377: writeback_congestion_wait: usec_timeout=100000 usec_delayed=100000
>
> The root cause is, the dirty threshold is knocked down a lot by the
> memory hog task. Fixed by using global_dirty_limit which decreases

AFAIUC, memory hog task, you mean it consumes lots of anon pages, not file-backed pages.
So global_dirty_limits would get small available_memory by global_reclaimable_pages
so that it ends up dirty threshold is knocked down.

Could you confirm my understanding?

Thanks.

> gradually on such events and can guarantee we stay above (the also
> decreasing) nr_dirty in the progress of following down to the new
> dirty threshold.
>
> Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
> ---
> mm/page-writeback.c | 1 +
> 1 file changed, 1 insertion(+)
>
> --- linux.orig/mm/page-writeback.c 2012-03-02 14:05:01.633763187 +0800
> +++ linux/mm/page-writeback.c 2012-03-02 14:11:52.929772962 +0800
> @@ -1472,6 +1472,7 @@ void throttle_vm_writeout(gfp_t gfp_mask
>
> for ( ; ; ) {
> global_dirty_limits(&background_thresh, &dirty_thresh);
> + dirty_thresh = hard_dirty_limit(dirty_thresh);
>
> /*
> * Boost the allowable dirty threshold a bit for page


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