Kernel version is 2.6.9, but I see no updates to this function in BK-current.
How is total_scanned ever updated? AFAICT it is always zero.
In mm/vmscan.c:balance_pgdat(), there are these references to total_scanned
(missing whitepace indicated by "^"):
977: int total_scanned, total_reclaimed;
983: total_scanned = 0;
1076: if (total_scanned > SWAP_CLUSTER_MAX * 2 &&
1077: total_scanned > total_reclaimed+total_reclaimed/2)
^ ^ ^ ^
1088: if (total_scanned && priority < DEF_PRIORITY - 2)
Could this be part of the problems with reclaim? Or have I missed something?
--Chuck Ebbert 06-Nov-04 14:15:21
Chuck Ebbert <[email protected]> wrote:
>
> Kernel version is 2.6.9, but I see no updates to this function in BK-current.
> How is total_scanned ever updated? AFAICT it is always zero.
It's a bug which was introduced months ago when we added struct
reclaim_state.
> In mm/vmscan.c:balance_pgdat(), there are these references to total_scanned
> (missing whitepace indicated by "^"):
>
>
> 977: int total_scanned, total_reclaimed;
>
> 983: total_scanned = 0;
>
> 1076: if (total_scanned > SWAP_CLUSTER_MAX * 2 &&
> 1077: total_scanned > total_reclaimed+total_reclaimed/2)
> ^ ^ ^ ^
>
> 1088: if (total_scanned && priority < DEF_PRIORITY - 2)
>
>
> Could this be part of the problems with reclaim? Or have I missed something?
I had a patch which fixes it in -mm for a while. It does increase the
number of pages which are reclaimed via direct reclaim and decreases the
number of pages which are reclaimed by kswapd. As one would expect from
throttling kswapd. This seems undesirable.
I'm leaving this alone until it can be demonstrated that fixing it improves
kernel behaviour in some manner.
Andrew Morton wrote:
> I'm leaving this alone until it can be demonstrated that fixing it improves
> kernel behaviour in some manner.
How about applying this patch so nobody else will be confused?
diff -ur bk-current/mm/vmscan.c edited/mm/vmscan.c
--- bk-current/mm/vmscan.c 2004-11-06 23:02:48.691160680 -0500
+++ edited/mm/vmscan.c 2004-11-06 23:13:02.636826752 -0500
@@ -1071,10 +1071,13 @@
/*
* If we've done a decent amount of scanning and
* the reclaim ratio is low, start doing writepage
- * even in laptop mode
+ * even in laptop mode.
+ * NOTE: total_scanned is always zero; this code
+ * does nothing. Reactivating it has not been
+ * shown to be helpful at the moment.
*/
if (total_scanned > SWAP_CLUSTER_MAX * 2 &&
- total_scanned > total_reclaimed+total_reclaimed/2)
+ total_scanned > total_reclaimed + total_reclaimed / 2)
sc.may_writepage = 1;
}
if (nr_pages && to_free > total_reclaimed)
@@ -1084,6 +1087,7 @@
/*
* OK, kswapd is getting into trouble. Take a nap, then take
* another pass across the zones.
+ * NOTE: total_scanned is always zero. See above.
*/
if (total_scanned && priority < DEF_PRIORITY - 2)
blk_congestion_wait(WRITE, HZ/10);
--Chuck Ebbert 06-Nov-04 23:35:37
On Sat, Nov 06, 2004 at 04:11:14PM -0800, Andrew Morton wrote:
> Chuck Ebbert <[email protected]> wrote:
> >
> > Kernel version is 2.6.9, but I see no updates to this function in BK-current.
> > How is total_scanned ever updated? AFAICT it is always zero.
>
> It's a bug which was introduced months ago when we added struct
> reclaim_state.
>
> > In mm/vmscan.c:balance_pgdat(), there are these references to total_scanned
> > (missing whitepace indicated by "^"):
> >
> >
> > 977: int total_scanned, total_reclaimed;
> >
> > 983: total_scanned = 0;
> >
> > 1076: if (total_scanned > SWAP_CLUSTER_MAX * 2 &&
> > 1077: total_scanned > total_reclaimed+total_reclaimed/2)
> > ^ ^ ^ ^
> >
> > 1088: if (total_scanned && priority < DEF_PRIORITY - 2)
> >
> >
> > Could this be part of the problems with reclaim? Or have I missed something?
>
> I had a patch which fixes it in -mm for a while. It does increase the
> number of pages which are reclaimed via direct reclaim and decreases the
> number of pages which are reclaimed by kswapd. As one would expect from
> throttling kswapd. This seems undesirable.
Hi Andrew,
Do you have any numbers to backup the claim "It does increase the
number of pages which are reclaimed via direct reclaim and decreases the
number of pages which are reclaimed by kswapd", please?
Because linux-2.6.10-rc1-mm2 (and 2.6.9) completly ignores sc->may_writepage
under normal operation, its only used when laptop_mode is on:
if (laptop_mode && !sc->may_writepage)
goto keep_locked;
Is this intentional ???
> I'm leaving this alone until it can be demonstrated that fixing it improves
> kernel behaviour in some manner.
I dont see it working at all?
I'll see if I find time to do some tests.. (my usual disclaimer).
Marcelo Tosatti <[email protected]> wrote:
>
> On Sat, Nov 06, 2004 at 04:11:14PM -0800, Andrew Morton wrote:
> > Chuck Ebbert <[email protected]> wrote:
> > >
> > > Kernel version is 2.6.9, but I see no updates to this function in BK-current.
> > > How is total_scanned ever updated? AFAICT it is always zero.
> >
> > It's a bug which was introduced months ago when we added struct
> > reclaim_state.
> >
> > > In mm/vmscan.c:balance_pgdat(), there are these references to total_scanned
> > > (missing whitepace indicated by "^"):
> > >
> > >
> > > 977: int total_scanned, total_reclaimed;
> > >
> > > 983: total_scanned = 0;
> > >
> > > 1076: if (total_scanned > SWAP_CLUSTER_MAX * 2 &&
> > > 1077: total_scanned > total_reclaimed+total_reclaimed/2)
> > > ^ ^ ^ ^
> > >
> > > 1088: if (total_scanned && priority < DEF_PRIORITY - 2)
> > >
> > >
> > > Could this be part of the problems with reclaim? Or have I missed something?
> >
> > I had a patch which fixes it in -mm for a while. It does increase the
> > number of pages which are reclaimed via direct reclaim and decreases the
> > number of pages which are reclaimed by kswapd. As one would expect from
> > throttling kswapd. This seems undesirable.
>
> Hi Andrew,
>
> Do you have any numbers to backup the claim "It does increase the
> number of pages which are reclaimed via direct reclaim and decreases the
> number of pages which are reclaimed by kswapd", please?
Run a workload and watch /proc/vmstat. iirc, the one-line total_scanned
fix takes the kswapd-vs-direct reclaim rate from 1:1 to 1:3 or thereabouts.
> Because linux-2.6.10-rc1-mm2 (and 2.6.9) completly ignores sc->may_writepage
> under normal operation, its only used when laptop_mode is on:
>
> if (laptop_mode && !sc->may_writepage)
> goto keep_locked;
>
> Is this intentional ???
yup. In laptop mode we try to scan further to find a clean page rather
than spinning up the disk for a writepage.
> > I'm leaving this alone until it can be demonstrated that fixing it improves
> > kernel behaviour in some manner.
>
> I dont see it working at all?
>
There's lots of useful info in /proc/vmstat.
On Tue, Nov 09, 2004 at 11:36:20AM -0800, Andrew Morton wrote:
> Marcelo Tosatti <[email protected]> wrote:
> >
> > On Sat, Nov 06, 2004 at 04:11:14PM -0800, Andrew Morton wrote:
> > > Chuck Ebbert <[email protected]> wrote:
> > > >
> > > > Kernel version is 2.6.9, but I see no updates to this function in BK-current.
> > > > How is total_scanned ever updated? AFAICT it is always zero.
> > >
> > > It's a bug which was introduced months ago when we added struct
> > > reclaim_state.
> > >
> > > > In mm/vmscan.c:balance_pgdat(), there are these references to total_scanned
> > > > (missing whitepace indicated by "^"):
> > > >
> > > >
> > > > 977: int total_scanned, total_reclaimed;
> > > >
> > > > 983: total_scanned = 0;
> > > >
> > > > 1076: if (total_scanned > SWAP_CLUSTER_MAX * 2 &&
> > > > 1077: total_scanned > total_reclaimed+total_reclaimed/2)
> > > > ^ ^ ^ ^
> > > >
> > > > 1088: if (total_scanned && priority < DEF_PRIORITY - 2)
> > > >
> > > >
> > > > Could this be part of the problems with reclaim? Or have I missed something?
> > >
> > > I had a patch which fixes it in -mm for a while. It does increase the
> > > number of pages which are reclaimed via direct reclaim and decreases the
> > > number of pages which are reclaimed by kswapd. As one would expect from
> > > throttling kswapd. This seems undesirable.
> >
> > Hi Andrew,
> >
> > Do you have any numbers to backup the claim "It does increase the
> > number of pages which are reclaimed via direct reclaim and decreases the
> > number of pages which are reclaimed by kswapd", please?
>
> Run a workload and watch /proc/vmstat. iirc, the one-line total_scanned
> fix takes the kswapd-vs-direct reclaim rate from 1:1 to 1:3 or thereabouts.
You're talking about laptop_mode ONLY, then?
How can that have any effect if may_writepage is ignored if !laptop_mode?
About /proc/vmstat - each output is huge - do you actually read those?
We need a vmstat like tool for that information to be readable.
> > Because linux-2.6.10-rc1-mm2 (and 2.6.9) completly ignores sc->may_writepage
> > under normal operation, its only used when laptop_mode is on:
> >
> > if (laptop_mode && !sc->may_writepage)
> > goto keep_locked;
> >
> > Is this intentional ???
>
> yup. In laptop mode we try to scan further to find a clean page rather
> than spinning up the disk for a writepage.
It might be interesting to use sc->may_writepage independantly of
laptop mode (ie make kswapd only writeout pages if the reclaim ratio
is low).
> > > I'm leaving this alone until it can be demonstrated that fixing it improves
> > > kernel behaviour in some manner.
> >
> > I dont see it working at all?
> >
>
> There's lots of useful info in /proc/vmstat.
I dont care much about laptop mode.
> > > > I had a patch which fixes it in -mm for a while. It does increase the
> > > > number of pages which are reclaimed via direct reclaim and decreases the
> > > > number of pages which are reclaimed by kswapd. As one would expect from
> > > > throttling kswapd. This seems undesirable.
> > >
> > > Hi Andrew,
> > >
> > > Do you have any numbers to backup the claim "It does increase the
> > > number of pages which are reclaimed via direct reclaim and decreases the
> > > number of pages which are reclaimed by kswapd", please?
> >
> > Run a workload and watch /proc/vmstat. iirc, the one-line total_scanned
> > fix takes the kswapd-vs-direct reclaim rate from 1:1 to 1:3 or thereabouts.
>
> You're talking about laptop_mode ONLY, then?
No, not at all.
If we restore the total_scanned logic then kswapd will throttle itself, as
designed. Regardless of laptop_mode. I did that, and monitored the page
scanning and reclaim rates under various workloads. I observed that with
the fix in place, kswapd performed less page reclaim and direct-reclaim
performed more reclaim. And I wasn't able to demonstrate any benchmark
improvements with the fix in place, so things are left as they are.
> How can that have any effect if may_writepage is ignored if !laptop_mode?
This is to do with kswapd throttling. If we put kswapd to sleep more
often, it does less scanning and reclaiming.
> About /proc/vmstat - each output is huge - do you actually read those?
yup.
cat /proc/vmstat > /tmp/1
run workload
cat /proc/vmstat > /tmp/2
analyse /tmp/1 and /tmp/2
> We need a vmstat like tool for that information to be readable.
Would be nice.
> > > Because linux-2.6.10-rc1-mm2 (and 2.6.9) completly ignores sc->may_writepage
> > > under normal operation, its only used when laptop_mode is on:
> > >
> > > if (laptop_mode && !sc->may_writepage)
> > > goto keep_locked;
> > >
> > > Is this intentional ???
> >
> > yup. In laptop mode we try to scan further to find a clean page rather
> > than spinning up the disk for a writepage.
>
> It might be interesting to use sc->may_writepage independantly of
> laptop mode (ie make kswapd only writeout pages if the reclaim ratio
> is low).
sure.
On Tue, Nov 09, 2004 at 01:40:32PM -0800, Andrew Morton wrote:
> > > > > I had a patch which fixes it in -mm for a while. It does increase the
> > > > > number of pages which are reclaimed via direct reclaim and decreases the
> > > > > number of pages which are reclaimed by kswapd. As one would expect from
> > > > > throttling kswapd. This seems undesirable.
> > > >
> > > > Hi Andrew,
> > > >
> > > > Do you have any numbers to backup the claim "It does increase the
> > > > number of pages which are reclaimed via direct reclaim and decreases the
> > > > number of pages which are reclaimed by kswapd", please?
> > >
> > > Run a workload and watch /proc/vmstat. iirc, the one-line total_scanned
> > > fix takes the kswapd-vs-direct reclaim rate from 1:1 to 1:3 or thereabouts.
> >
> > You're talking about laptop_mode ONLY, then?
>
> No, not at all.
>
> If we restore the total_scanned logic then kswapd will throttle itself, as
> designed. Regardless of laptop_mode. I did that, and monitored the page
> scanning and reclaim rates under various workloads. I observed that with
> the fix in place, kswapd performed less page reclaim and direct-reclaim
> performed more reclaim. And I wasn't able to demonstrate any benchmark
> improvements with the fix in place, so things are left as they are.
Ah, OK, I understand what you mean. I was thinking about sc->may_writepage
only and its effects on shrink_list/pageout.
You remind me about the self throttling (blk_congestion_wait).
It makes sense now.
Andrea noted that blk_congestion_wait waits on IO which is not generated by
reclaim - which is indeed a bad thing - it should only wait on IO which the
VM itself has started.
> > How can that have any effect if may_writepage is ignored if !laptop_mode?
>
> This is to do with kswapd throttling. If we put kswapd to sleep more
> often, it does less scanning and reclaiming.
OK!
> > About /proc/vmstat - each output is huge - do you actually read those?
>
> yup.
>
> cat /proc/vmstat > /tmp/1
> run workload
> cat /proc/vmstat > /tmp/2
> analyse /tmp/1 and /tmp/2
Will do that more often. :)
> > We need a vmstat like tool for that information to be readable.
>
> Would be nice.
I've been thinking on doing a Python based tool someday.
> > > > Because linux-2.6.10-rc1-mm2 (and 2.6.9) completly ignores sc->may_writepage
> > > > under normal operation, its only used when laptop_mode is on:
> > > >
> > > > if (laptop_mode && !sc->may_writepage)
> > > > goto keep_locked;
> > > >
> > > > Is this intentional ???
> > >
> > > yup. In laptop mode we try to scan further to find a clean page rather
> > > than spinning up the disk for a writepage.
> >
> > It might be interesting to use sc->may_writepage independantly of
> > laptop mode (ie make kswapd only writeout pages if the reclaim ratio
> > is low).
>
> sure.
Another related thing I noted this afternoon is that right now kswapd will
always block on full queues:
static int may_write_to_queue(struct backing_dev_info *bdi)
{
if (current_is_kswapd())
return 1;
if (current_is_pdflush()) /* This is unlikely, but why not... */
return 1;
if (!bdi_write_congested(bdi))
return 1;
if (bdi == current->backing_dev_info)
return 1;
return 0;
}
We should make kswapd use the "bdi_write_congested" information and avoid
blocking on full queues. It should improve performance on multi-device
systems with intense VM loads.
Maybe something along the lines
"if the reclaim ratio is high, do not writepage"
"if the reclaim ratio is below high, writepage but not block"
"if the reclaim ratio is low, writepage and block"
Marcelo Tosatti <[email protected]> wrote:
>
> ...
> > > You're talking about laptop_mode ONLY, then?
> >
> > No, not at all.
> >
> > If we restore the total_scanned logic then kswapd will throttle itself, as
> > designed. Regardless of laptop_mode. I did that, and monitored the page
> > scanning and reclaim rates under various workloads. I observed that with
> > the fix in place, kswapd performed less page reclaim and direct-reclaim
> > performed more reclaim. And I wasn't able to demonstrate any benchmark
> > improvements with the fix in place, so things are left as they are.
>
> Ah, OK, I understand what you mean. I was thinking about sc->may_writepage
> only and its effects on shrink_list/pageout.
>
> You remind me about the self throttling (blk_congestion_wait).
> It makes sense now.
>
> Andrea noted that blk_congestion_wait waits on IO which is not generated by
> reclaim - which is indeed a bad thing - it should only wait on IO which the
> VM itself has started.
Yes, blk_congestion_wait() is very approximate. It was always intended
that it be replaced by wakeups from end_page_writeback(), directed to
waitqueues which correspond to the classzones to which the page belongs.
That way, page reclaiming processes can throttle precisely upon I/O
completion against pages which are useful to them.
But I've never seen a report of a problem which would be solved by such a
change, and so the cost of delivering multiple wakeups at
end_page_writeback() doesn't seem justified thus far.
> ...
> Another related thing I noted this afternoon is that right now kswapd will
> always block on full queues:
>
> static int may_write_to_queue(struct backing_dev_info *bdi)
> {
> if (current_is_kswapd())
> return 1;
> if (current_is_pdflush()) /* This is unlikely, but why not... */
> return 1;
> if (!bdi_write_congested(bdi))
> return 1;
> if (bdi == current->backing_dev_info)
> return 1;
> return 0;
> }
>
> We should make kswapd use the "bdi_write_congested" information and avoid
> blocking on full queues. It should improve performance on multi-device
> systems with intense VM loads.
>
> Maybe something along the lines
>
> "if the reclaim ratio is high, do not writepage"
> "if the reclaim ratio is below high, writepage but not block"
> "if the reclaim ratio is low, writepage and block"
It used to do that, but it was taken out. gack, brain-strain. umm, dig,
dig. Here you go:
The `low latency page reclaim' design works by preventing page
allocators from blocking on request queues (and by preventing them from
blocking against writeback of individual pages, but that is immaterial
here).
This has a problem under some situations. pdflush (or a write(2)
caller) could be saturating the queue with highmem pages. This
prevents anyone from writing back ZONE_NORMAL pages. We end up doing
enormous amounts of scenning.
A test case is to mmap(MAP_SHARED) almost all of a 4G machine's memory,
then kill the mmapping applications. The machine instantly goes from
0% of memory dirty to 95% or more. pdflush kicks in and starts writing
the least-recently-dirtied pages, which are all highmem. The queue is
congested so nobody will write back ZONE_NORMAL pages. kswapd chews
50% of the CPU scanning past dirty ZONE_NORMAL pages and page reclaim
efficiency (pages_reclaimed/pages_scanned) falls to 2%.
So this patch changes the policy for kswapd. kswapd may use all of a
request queue, and is prepared to block on request queues.
What will now happen in the above scenario is:
1: The page alloctor scans some pages, fails to reclaim enough
memory and takes a nap in blk_congetion_wait().
2: kswapd() will scan the ZONE_NORMAL LRU and will start writing
back pages. (These pages will be rotated to the tail of the
inactive list at IO-completion interrupt time).
This writeback will saturate the queue with ZONE_NORMAL pages.
Conveniently, pdflush will avoid the congested queues. So we end up
writing the correct pages.
In this test, kswapd CPU utilisation falls from 50% to 2%, page reclaim
efficiency rises from 2% to 40% and things are generally a lot happier.
The downside is that kswapd may now do a lot less page reclaim,
increasing page allocation latency, causing more direct reclaim,
increasing lock contention in the VM, etc. But I have not been able to
demonstrate that in testing.
The other problem is that there is only one kswapd, and there are lots
of disks. That is a generic problem - without being able to co-opt
user processes we don't have enough threads to keep lots of disks saturated.
One fix for this would be to add an additional "really congested"
threshold in the request queues, so kswapd can still perform
nonblocking writeout. This gives kswapd priority over pdflush while
allowing kswapd to feed many disk queues. I doubt if this will be
called for.
include/linux/swap.h | 6 ++++++
mm/vmscan.c | 21 +++++++++++++++------
2 files changed, 21 insertions(+), 6 deletions(-)
--- 25/mm/vmscan.c~blocking-kswapd Sat Dec 21 16:24:37 2002
+++ 25-akpm/mm/vmscan.c Sat Dec 21 16:24:37 2002
@@ -204,6 +204,19 @@ static inline int is_page_cache_freeable
return page_count(page) - !!PagePrivate(page) == 2;
}
+static int may_write_to_queue(struct backing_dev_info *bdi)
+{
+ if (current_is_kswapd())
+ return 1;
+ if (current_is_pdflush()) /* This is unlikely, but why not... */
+ return 1;
+ if (!bdi_write_congested(bdi))
+ return 1;
+ if (bdi == current->backing_dev_info)
+ return 1;
+ return 0;
+}
+
/*
* shrink_list returns the number of reclaimed pages
*/
@@ -303,8 +316,6 @@ shrink_list(struct list_head *page_list,
* See swapfile.c:page_queue_congested().
*/
if (PageDirty(page)) {
- struct backing_dev_info *bdi;
-
if (!is_page_cache_freeable(page))
goto keep_locked;
if (!mapping)
@@ -313,9 +324,7 @@ shrink_list(struct list_head *page_list,
goto activate_locked;
if (!may_enter_fs)
goto keep_locked;
- bdi = mapping->backing_dev_info;
- if (bdi != current->backing_dev_info &&
- bdi_write_congested(bdi))
+ if (!may_write_to_queue(mapping->backing_dev_info))
goto keep_locked;
write_lock(&mapping->page_lock);
if (test_clear_page_dirty(page)) {
@@ -424,7 +433,7 @@ keep:
if (pagevec_count(&freed_pvec))
__pagevec_release_nonlru(&freed_pvec);
mod_page_state(pgsteal, ret);
- if (current->flags & PF_KSWAPD)
+ if (current_is_kswapd())
mod_page_state(kswapd_steal, ret);
mod_page_state(pgactivate, pgactivate);
return ret;
--- 25/include/linux/swap.h~blocking-kswapd Sat Dec 21 16:24:37 2002
+++ 25-akpm/include/linux/swap.h Sat Dec 21 16:24:37 2002
@@ -7,6 +7,7 @@
#include <linux/linkage.h>
#include <linux/mmzone.h>
#include <linux/list.h>
+#include <linux/sched.h>
#include <asm/atomic.h>
#include <asm/page.h>
@@ -14,6 +15,11 @@
#define SWAP_FLAG_PRIO_MASK 0x7fff
#define SWAP_FLAG_PRIO_SHIFT 0
+static inline int current_is_kswapd(void)
+{
+ return current->flags & PF_KSWAPD;
+}
+
/*
* MAX_SWAPFILES defines the maximum number of swaptypes: things which can
* be swapped to. The swap type and the offset into that swap type are
_
Andrew Morton <[email protected]> wrote:
> There's lots of useful info in /proc/vmstat.
And the documentation on these fields is the source code itself, right? :)
The nr_dirty field seems kind of useless -- why not have nr_dirtied
and nr_cleaned instead? Analysis tools can subtract them to get nr_dirty.
Or is there some other field that shows the nr of pages being dirtied?
--Chuck Ebbert 09-Nov-04 22:13:44
Marcelo Tosatti writes:
[...]
>
> Another related thing I noted this afternoon is that right now kswapd will
> always block on full queues:
>
> static int may_write_to_queue(struct backing_dev_info *bdi)
> {
> if (current_is_kswapd())
> return 1;
> if (current_is_pdflush()) /* This is unlikely, but why not... */
> return 1;
> if (!bdi_write_congested(bdi))
> return 1;
> if (bdi == current->backing_dev_info)
> return 1;
> return 0;
> }
>
> We should make kswapd use the "bdi_write_congested" information and avoid
> blocking on full queues. It should improve performance on multi-device
> systems with intense VM loads.
This will have following undesirable side effect: if
may_write_to_queue() returns false, page is not paged out, instead it is
thrown to the head of the inactive queue, thus destroying "LRU
ordering", shrink_list() will dive deeper into inactive list, reclaiming
hotter pages.
It's OK to accidentially skip pageout in direct reclaim path, because
- we hope most pageout is done by kswapd, and
- we don't want __alloc_pages() to stall
but _something_ in the kernel should take a pain of actually writing
pages out in LRU order.
>
> Maybe something along the lines
>
> "if the reclaim ratio is high, do not writepage"
> "if the reclaim ratio is below high, writepage but not block"
> "if the reclaim ratio is low, writepage and block"
If kswapd blocking is a concern, inactive list scanning should be
decoupled from actual page-out (a la Solaris): kswapd queues pages to
the yet another kernel thread that calls pageout().
I played with this idea (see
http://nikita.w3.to/code/patches/2-6-10-rc1/async-writepage.txt note
that async_writepage() has to be adjusted to work for kswapd), but while
in some cases (large concurrent builds) it does provide a benefit, in
other cases (heavy write through mmap) it makes throughput slightly
worse.
Besides, this doesn't completely avoid the problem of destroying LRU
ordering, as kswapd still proceeds further through inactive list while
pages are sent out asynchronously.
Nikita.