On Thu 05-06-14 06:33:40, Felipe Contreras wrote:
> Hi,
Hi,
> For a while I've noticed that my machine bogs down in certain
> situations, usually while doing heavy I/O operations, it is not just the
> I/O operations, but everything, including the graphical interface, even
> the mouse pointer.
>
> As far as I can recall this did not happen in the past.
>
> I noticed this specially on certain operations, for example updating a
> a game on Steam (to an exteranl USB 3.0 device), or copying TV episodes
> to a USB memory stick (probably flash-based).
We had a similar report for opensuse. The common part was that there was
an IO to a slow USB device going on.
> Today I decided to finally hunt down the problem, so I created a
> synthetic test that basically consists on copying a bunch of files from
> one drive to another (from an SSD to an external USB 3.0). This is
> pretty similar to what I noticed; the graphical interface slows down.
>
> Then I bisected the issue and it turns out that indeed it wasn't
> happening in the past, it started happening in v3.11, and it was
> triggered by this commit:
>
> e2be15f (mm: vmscan: stall page reclaim and writeback pages based on
> dirty/writepage pages encountered)
>
> Then I went back to the latest stable version (v3.14.5), and commented
> out the line I think is causing the slow down:
>
> if (nr_unqueued_dirty == nr_taken || nr_immediate)
> congestion_wait(BLK_RW_ASYNC, HZ/10);
Yes, I came to the same check. I didn't have any confirmation yet so
thanks for your confirmation. I've suggested to reduce this
congestion_wait only to kswapd:
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 32c661d66a45..ef6a1c0e788c 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1566,7 +1566,7 @@ shrink_inactive_list(unsigned long nr_to_scan, struct
lruvec *lruvec,
* implies that pages are cycling through the LRU faster than
* they are written so also forcibly stall.
*/
- if (nr_unqueued_dirty == nr_taken || nr_immediate)
+ if ((nr_unqueued_dirty == nr_taken || nr_immediate) &&
current_is_kswapd())
congestion_wait(BLK_RW_ASYNC, HZ/10);
}
But I am still not sure whether this is the right way to fix it. Direct
reclaimers can be throttled later on wait_iff_congested. I guess the
original intention was to throttle kswapd to not scan LRU full of dirty
pages like crazy. So I think it makes some sense to reduce the
congestion_wait only to kswapd.
> After that I don't notice the slow down any more.
>
> Anybody has any ideas how to fix the issue properly?
>
> --
> Felipe Contreras
>
> --
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--
Michal Hocko
SUSE Labs
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