Messages in this thread | | | Date | Fri, 23 Jul 2010 16:33:15 +0800 | From | Wu Fengguang <> | Subject | Re: [PATCH 7/8] writeback: sync old inodes first in background writeback |
| |
Hi Mel,
On Thu, Jul 22, 2010 at 05:42:09PM +0800, Mel Gorman wrote: > On Thu, Jul 22, 2010 at 04:52:10PM +0800, Wu Fengguang wrote: > > > Some insight on how the other writeback changes that are being floated > > > around might affect the number of dirty pages reclaim encounters would also > > > be helpful. > > > > Here is an interesting related problem about the wait_on_page_writeback() call > > inside shrink_page_list(): > > > > http://lkml.org/lkml/2010/4/4/86
I guess you've got the answers from the above thread, anyway here is the brief answers to your questions.
> > > > The problem is, wait_on_page_writeback() is called too early in the > > direct reclaim path, which blocks many random/unrelated processes when > > some slow (USB stick) writeback is on the way. > > > > A simple dd can easily create a big range of dirty pages in the LRU > > list. Therefore priority can easily go below (DEF_PRIORITY - 2) in a > > typical desktop, which triggers the lumpy reclaim mode and hence > > wait_on_page_writeback(). > > > > Lumpy reclaim is for high-order allocations. A simple dd should not be > triggering it regularly unless there was a lot of forking going on at the > same time.
dd could create the dirty file fast enough, so that no other processes are injecting pages into the LRU lists besides dd itself. So it's creating a large range of hard-to-reclaim LRU pages which will trigger this code
+ else if (sc->order && priority < DEF_PRIORITY - 2) + lumpy_reclaim = 1;
> Also, how would a random or unrelated process get blocked on > writeback unless they were also doing high-order allocations? What was the > source of the high-order allocations?
sc->order is 1 on fork().
Thanks, Fengguang
| |