Re: [Qemu-devel] Windows VM slow boot

[Mel Gorman]

On Wed, Sep 12, 2012 at 11:56:59AM +0100, Richard Davies wrote:
> [ adding linux-mm - previously at http://marc.info/?t=134511509400003 ]
> 
> Hi Rik,
> 

I'm not Rik but hi anyway.

> Since qemu-kvm 1.2.0 and Linux 3.6.0-rc5 came out, I thought that I would
> retest with these.
> 

Ok. 3.6.0-rc5 contains [c67fe375: mm: compaction: Abort async compaction
if locks are contended or taking too long] that should have helped mitigate
some of the lock contention problem but not all of it as we'll see later.

> The typical symptom now appears to be that the Windows VMs boot reasonably
> fast,

I see that this is an old-ish bug but I did not read the full history.
Is it now booting faster than 3.5.0 was? I'm asking because I'm
interested to see if commit c67fe375 helped your particular case.

> but then there is high CPU use and load for many minutes afterwards -
> the high CPU use is both for the qemu-kvm processes themselves and also for
> % sys.
> 

Ok, I cannot comment on the userspace portion of things but the kernel
portion still indicates that there is a high percentage of time on what
appears to be lock contention.

> I attach a perf report which seems to show that the high CPU use is in the
> memory manager.
> 

A follow-on from commit c67fe375 was the following patch (author cc'd)
which addresses lock contention in isolate_migratepages_range where your
perf report indicates that we're spending 95% of the time. Would you be
willing to test it please?

---8<---
From: Shaohua Li <address@hidden>
Subject: mm: compaction: check lock contention first before taking lock

isolate_migratepages_range will take zone->lru_lock first and check if the
lock is contented, if yes, it will release the lock.  This isn't
efficient.  If the lock is truly contented, a lock/unlock pair will
increase the lock contention.  We'd better check if the lock is contended
first.  compact_trylock_irqsave perfectly meets the requirement.

Signed-off-by: Shaohua Li <address@hidden>
Acked-by: Mel Gorman <address@hidden>
Acked-by: Minchan Kim <address@hidden>
Signed-off-by: Andrew Morton <address@hidden>
---

 mm/compaction.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff -puN 
mm/compaction.c~mm-compaction-check-lock-contention-first-before-taking-lock 
mm/compaction.c
--- 
a/mm/compaction.c~mm-compaction-check-lock-contention-first-before-taking-lock
+++ a/mm/compaction.c
@@ -349,8 +349,9 @@ isolate_migratepages_range(struct zone *
 
        /* Time to isolate some pages for migration */
        cond_resched();
-       spin_lock_irqsave(&zone->lru_lock, flags);
-       locked = true;
+       locked = compact_trylock_irqsave(&zone->lru_lock, &flags, cc);
+       if (!locked)
+               return 0;
        for (; low_pfn < end_pfn; low_pfn++) {
                struct page *page;