On Fri, 2007-11-02 at 10:21 +0800, Fengguang Wu wrote:
> Interestingly, no background_writeout() appears, but only
> balance_dirty_pages() and wb_kupdate. Obviously wb_kupdate won't
> block the process.
Yeah, the background threshold is not (yet) scaled. So it can happen
that the bdi_dirty limit is below the background limit.
I'm curious though as to these stalls, though, I can't seem to think of
what goes wrong.. esp since most writeback seems to happen from pdflush.
(or I'm totally misreading it - quite a possible as I'm still recovering
from a serious cold and not all the green stuff has yet figured out its
proper place wrt brain cells 'n stuff)
I still have this patch floating around:
---
Subject: mm: speed up writeback ramp-up on clean systems
We allow violation of bdi limits if there is a lot of room on the
system. Once we hit half the total limit we start enforcing bdi limits
and bdi ramp-up should happen. Doing it this way avoids many small
writeouts on an otherwise idle system and should also speed up the
ramp-up.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
---
mm/page-writeback.c | 19 +++++++++++++++++--
1 file changed, 17 insertions(+), 2 deletions(-)
Index: linux-2.6/mm/page-writeback.c
===================================================================
--- linux-2.6.orig/mm/page-writeback.c 2007-09-28 10:08:33.937415368 +0200
+++ linux-2.6/mm/page-writeback.c 2007-09-28 10:54:26.018247516 +0200
@@ -355,8 +355,8 @@ get_dirty_limits(long *pbackground, long
*/
static void balance_dirty_pages(struct address_space *mapping)
{
- long bdi_nr_reclaimable;
- long bdi_nr_writeback;
+ long nr_reclaimable, bdi_nr_reclaimable;
+ long nr_writeback, bdi_nr_writeback;
long background_thresh;
long dirty_thresh;
long bdi_thresh;
@@ -376,11 +376,26 @@ static void balance_dirty_pages(struct a
get_dirty_limits(&background_thresh, &dirty_thresh,
&bdi_thresh, bdi);
+
+ nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
+ global_page_state(NR_UNSTABLE_NFS);
+ nr_writeback = global_page_state(NR_WRITEBACK);
+
bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
+
if (bdi_nr_reclaimable + bdi_nr_writeback <= bdi_thresh)
break;
+ /*
+ * Throttle it only when the background writeback cannot
+ * catch-up. This avoids (excessively) small writeouts
+ * when the bdi limits are ramping up.
+ */
+ if (nr_reclaimable + nr_writeback <
+ (background_thresh + dirty_thresh) / 2)
+ break;
+
if (!bdi->dirty_exceeded)
bdi->dirty_exceeded = 1;
-
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