Date: Wed, 8 Oct 2014 20:01:52 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Dmitry Sivachenko <trtrmitya@gmail.com> Cc: "hackers@freebsd.org" <hackers@freebsd.org> Subject: Re: mmap() question Message-ID: <20141008170152.GG2153@kib.kiev.ua> In-Reply-To: <5C10922E-7030-4C89-9FD3-DA770E462067@gmail.com> References: <95E0B821-BF9B-4EBF-A1E5-1DDCBB1C3D1B@gmail.com> <20131011051702.GE41229@kib.kiev.ua> <A5E3C0A2-F0D5-47B1-8992-4B9DA347C275@gmail.com> <20131012095919.GI41229@kib.kiev.ua> <5C10922E-7030-4C89-9FD3-DA770E462067@gmail.com>
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On Wed, Oct 08, 2014 at 07:14:45PM +0400, Dmitry Sivachenko wrote:
> On 12 ΟΛΤ. 2013 Η., at 13:59, Konstantin Belousov <kostikbel@gmail.com> wrote:
> >
> > I was not able to reproduce the situation locally. I even tried to start
> > a lot of threads accessing the mapped regions, to try to outrun the
> > pagedaemon. The user threads sleep on the disk read, while pagedaemon
> > has a lot of time to rebalance the queues. It might be a case when SSD
> > indeed makes a difference.
> >
> > Still, I see how this situation could appear. The code, which triggers
> > OOM, never fires if there is a free space in the swapfile, so the
> > absense of swap is neccessary condition to trigger the bug. Next, OOM
> > calculation does not account for a possibility that almost all pages on
> > the queues can be reused. It just fires if free pages depleted too much
> > or free target cannot be reached.
> >
> > IMO one of the possible solution is to account the queued pages in
> > addition to the swap space. This is not entirely accurate, since some
> > pages on the queues cannot be reused, at least transiently. Most precise
> > algorithm would count the hold and busy pages globally, and substract
> > this count from queues length, but it is probably too costly.
> >
> > Instead, I think we could rely on the numbers which are counted by
> > pagedaemon threads during the passes. Due to the transient nature of the
> > pagedaemon failures, this should be fine.
> >
> > Below is the prototype patch, against HEAD. It is not applicable to
> > stable, please use HEAD kernel for test.
>
>
>
> Hello,
>
> any chance to commit this patch?
>
I wrote something different after discussion with Alan, it instead measures
the pagedaemon progress. Unfortunately, I have some issues with auto-tuning
the detection. Could you try the patch below. Possibly, you would need
to change vm.pageout_oom_miss (either lower or raise, depending on the
load) to get the tuning. Bigger the value, more the time pagedaemon tries
to make any progress before declaring OOM.
diff --git a/sys/vm/vm_page.h b/sys/vm/vm_page.h
index f12b76c..251cf7b 100644
--- a/sys/vm/vm_page.h
+++ b/sys/vm/vm_page.h
@@ -227,6 +227,7 @@ struct vm_domain {
long vmd_segs; /* bitmask of the segments */
boolean_t vmd_oom;
int vmd_pass; /* local pagedaemon pass */
+ int vmd_oom_seq;
struct vm_page vmd_marker; /* marker for pagedaemon private use */
};
diff --git a/sys/vm/vm_pageout.c b/sys/vm/vm_pageout.c
index ca9d7f9..aa23eb8 100644
--- a/sys/vm/vm_pageout.c
+++ b/sys/vm/vm_pageout.c
@@ -120,7 +120,8 @@ static void vm_pageout(void);
static void vm_pageout_init(void);
static int vm_pageout_clean(vm_page_t);
static void vm_pageout_scan(struct vm_domain *vmd, int pass);
-static void vm_pageout_mightbe_oom(struct vm_domain *vmd, int pass);
+static void vm_pageout_mightbe_oom(struct vm_domain *vmd, int page_shortage,
+ int starting_page_shortage);
SYSINIT(pagedaemon_init, SI_SUB_KTHREAD_PAGE, SI_ORDER_FIRST, vm_pageout_init,
NULL);
@@ -157,6 +158,7 @@ int vm_pages_needed; /* Event on which pageout daemon sleeps */
int vm_pageout_deficit; /* Estimated number of pages deficit */
int vm_pageout_pages_needed; /* flag saying that the pageout daemon needs pages */
int vm_pageout_wakeup_thresh;
+static int vm_pageout_oom_seq = 24;
#if !defined(NO_SWAPPING)
static int vm_pageout_req_swapout; /* XXX */
@@ -216,6 +218,10 @@ static int pageout_lock_miss;
SYSCTL_INT(_vm, OID_AUTO, pageout_lock_miss,
CTLFLAG_RD, &pageout_lock_miss, 0, "vget() lock misses during pageout");
+SYSCTL_INT(_vm, OID_AUTO, pageout_oom_seq,
+ CTLFLAG_RW, &vm_pageout_oom_seq, 0,
+ "side-to-side calls to oom detector to start OOM");
+
#define VM_PAGEOUT_PAGE_COUNT 16
int vm_pageout_page_count = VM_PAGEOUT_PAGE_COUNT;
@@ -912,7 +918,8 @@ vm_pageout_scan(struct vm_domain *vmd, int pass)
vm_page_t m, next;
struct vm_pagequeue *pq;
vm_object_t object;
- int act_delta, addl_page_shortage, deficit, maxscan, page_shortage;
+ int act_delta, addl_page_shortage, deficit, maxscan;
+ int page_shortage, starting_page_shortage;
int vnodes_skipped = 0;
int maxlaunder;
int lockmode;
@@ -954,6 +961,7 @@ vm_pageout_scan(struct vm_domain *vmd, int pass)
page_shortage = vm_paging_target() + deficit;
} else
page_shortage = deficit = 0;
+ starting_page_shortage = page_shortage;
/*
* maxlaunder limits the number of dirty pages we flush per scan.
@@ -1329,6 +1337,15 @@ relock_queues:
(void)speedup_syncer();
/*
+ * If we are critically low on one of RAM or swap and low on
+ * the other, kill the largest process. However, we avoid
+ * doing this on the first pass in order to give ourselves a
+ * chance to flush out dirty vnode-backed pages and to allow
+ * active pages to be moved to the inactive queue and reclaimed.
+ */
+ vm_pageout_mightbe_oom(vmd, page_shortage, starting_page_shortage);
+
+ /*
* Compute the number of pages we want to try to move from the
* active queue to the inactive queue.
*/
@@ -1437,15 +1454,6 @@ relock_queues:
}
}
#endif
-
- /*
- * If we are critically low on one of RAM or swap and low on
- * the other, kill the largest process. However, we avoid
- * doing this on the first pass in order to give ourselves a
- * chance to flush out dirty vnode-backed pages and to allow
- * active pages to be moved to the inactive queue and reclaimed.
- */
- vm_pageout_mightbe_oom(vmd, pass);
}
static int vm_pageout_oom_vote;
@@ -1456,18 +1464,36 @@ static int vm_pageout_oom_vote;
* failed to reach free target is premature.
*/
static void
-vm_pageout_mightbe_oom(struct vm_domain *vmd, int pass)
+vm_pageout_mightbe_oom(struct vm_domain *vmd, int page_shortage,
+ int starting_page_shortage)
{
int old_vote;
- if (pass <= 1 || !((swap_pager_avail < 64 && vm_page_count_min()) ||
- (swap_pager_full && vm_paging_target() > 0))) {
+ if (starting_page_shortage <= 0 || starting_page_shortage !=
+ page_shortage) {
+#if 0
+ if (vmd->vmd_oom_seq != 0)
+ printf("CLR oom_seq %d ps %d sps %d\n", vmd->vmd_oom_seq, page_shortage, starting_page_shortage);
+#endif
+ vmd->vmd_oom_seq = 0;
+ } else
+ vmd->vmd_oom_seq++;
+ if (vmd->vmd_oom_seq < vm_pageout_oom_seq) {
if (vmd->vmd_oom) {
vmd->vmd_oom = FALSE;
atomic_subtract_int(&vm_pageout_oom_vote, 1);
}
return;
}
+#if 0
+printf("OOM oom_seq %d ps %d sps %d\n", vmd->vmd_oom_seq, page_shortage, starting_page_shortage);
+#endif
+
+ /*
+ * Do not follow the call sequence until OOM condition is
+ * cleared.
+ */
+ vmd->vmd_oom_seq = 0;
if (vmd->vmd_oom)
return;
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