Re: regression in page writeback

Wu Fengguang
2009-09-29T00:15:16Z

On Mon, Sep 28, 2009 at 03:15:07PM +0800, Wu Fengguang wrote:
> On Mon, Sep 28, 2009 at 09:07:00AM +0800, Dave Chinner wrote:
> >
> > pageout is so horribly inefficient from an IO perspective it is not
> > funny. It is one of the reasons Linux sucks so much when under
> > memory pressure. It basically causes the system to do random 4k
> > writeback of dirty pages (and lumpy reclaim can make it
> > synchronous!).
> >
> > pageout needs an enema, and preferably it should defer to background
> > writeback to clean pages. background writeback will clean pages
> > much, much faster than the random crap that pageout spews at the
> > disk right now.
> >
> > Given that I can basically lock up my 2.6.30-based laptop for 10-15
> > minutes at a time with the disk running flat out in low memory
> > situations simply by starting to copy a large file(*), I think that
> > the way we currently handle dirty page writeback needs a bit of a
> > rethink.
> >
> > (*) I had this happen 4-5 times last week moving VM images around on
> > my laptop, and it involved the Linux VM switching between pageout
> > and swapping to make more memory available while the copy was was
> > hammering the same drive with dirty pages from foreground writeback.
> > It made for extremely fragmented files when the machine finally
> > recovered because of the non-sequential writeback patterns on the
> > single file being copied. You can't tell me that this is sane,
> > desirable behaviour, and this is the sort of problem that I want
> > sorted out. I don't beleive it can be fixed by maintaining the
> > number of uncoordinated, competing writeback mechanisms we currently
> > have.
>
> I imagined some lumpy pageout policy would help, but didn't realize
> it's such a severe problem that can happen in daily desktop workload..
>
> Below is a quick patch. Any comments?

Wow, it's much easier to reuse write_cache_pages for lumpy pageout :)

---
mm/page-writeback.c | 36 ++++++++++++++++++++++++------------
mm/shmem.c | 1 +
mm/vmscan.c | 6 ++++++
3 files changed, 31 insertions(+), 12 deletions(-)

--- linux.orig/mm/vmscan.c 2009-09-29 07:21:51.000000000 +0800
+++ linux/mm/vmscan.c 2009-09-29 07:46:59.000000000 +0800
@@ -344,6 +344,8 @@ typedef enum {
PAGE_CLEAN,
} pageout_t;

+#define LUMPY_PAGEOUT_PAGES (512 * 1024 / PAGE_CACHE_SIZE)
+
/*
* pageout is called by shrink_page_list() for each dirty page.
* Calls ->writepage().
@@ -408,6 +410,10 @@ static pageout_t pageout(struct page *pa
return PAGE_ACTIVATE;
}

+ wbc.range_start = (page->index + 1) << PAGE_CACHE_SHIFT;
+ wbc.nr_to_write = LUMPY_PAGEOUT_PAGES - 1;
+ generic_writepages(mapping, &wbc);
+
/*
* Wait on writeback if requested to. This happens when
* direct reclaiming a large contiguous area and the
--- linux.orig/mm/page-writeback.c 2009-09-29 07:33:13.000000000 +0800
+++ linux/mm/page-writeback.c 2009-09-29 08:10:39.000000000 +0800
@@ -799,6 +799,12 @@ retry:
if (nr_pages == 0)
break;

+ if (wbc->for_reclaim && done_index + nr_pages - 1 !=
+ pvec.pages[nr_pages - 1]->index) {
+ pagevec_release(&pvec);
+ break;
+ }
+
for (i = 0; i < nr_pages; i++) {
struct page *page = pvec.pages[i];

@@ -852,24 +858,30 @@ continue_unlock:
if (!clear_page_dirty_for_io(page))
goto continue_unlock;

+ /*
+ * active and unevictable pages will be checked at
+ * rotate time
+ */
+ if (wbc->for_reclaim)
+ SetPageReclaim(page);
+
ret = (*writepage)(page, wbc, data);
if (unlikely(ret)) {
if (ret == AOP_WRITEPAGE_ACTIVATE) {
unlock_page(page);
ret = 0;
- } else {
- /*
- * done_index is set past this page,
- * so media errors will not choke
- * background writeout for the entire
- * file. This has consequences for
- * range_cyclic semantics (ie. it may
- * not be suitable for data integrity
- * writeout).
- */
- done = 1;
- break;
}
+ /*
+ * done_index is set past this page,
+ * so media errors will not choke
+ * background writeout for the entire
+ * file. This has consequences for
+ * range_cyclic semantics (ie. it may
+ * not be suitable for data integrity
+ * writeout).
+ */
+ done = 1;
+ break;
}

if (nr_to_write > 0) {
--- linux.orig/mm/shmem.c 2009-09-29 08:07:22.000000000 +0800
+++ linux/mm/shmem.c 2009-09-29 08:08:02.000000000 +0800
@@ -1103,6 +1103,7 @@ unlock:
*/
swapcache_free(swap, NULL);
redirty:
+ wbc->pages_skipped++;
set_page_dirty(page);
if (wbc->for_reclaim)
return AOP_WRITEPAGE_ACTIVATE; /* Return with page locked */