Re: Kernel 2.6.8.1: swap storm of death - nr_requests > 1024 on swap partition

[Marcelo Tosatti <marcelo.tosatti@cyclades.com>]

On Mon, Aug 30, 2004 at 04:51:00PM -0700, Andrew Morton wrote:
> Marcelo Tosatti <marcelo.tosatti@cyclades.com> wrote:
> >
> > What you think of this, which tries to address your comments
> 
> Suggest you pass the scan_control structure down into pageout(), stick
> `inflight' into struct scan_control and use some flag in scan_control to

Done the scan_control modifications.

> ensure that we only throttle once per try_to_free_pages()/blaance_pgdat()
> pass.

Throttling once is enough

I added a 

+		 if (sc->throttled < 5) {
+                       blk_congestion_wait(WRITE, HZ/5);
+                       sc->throttled++;
+               }

To loop five times max per try_to_free_pages()/balance_pgdat().

Because only one blk_congestion_wait(WRITE, HZ/5)
makes my 64MB boot testcase with 8192 nr_requests fail. The OOM killer
triggers prematurely.

> See, page reclaim is now, as much as possible, "batched".  Think of it as
> operating in units of 32 pages at a time.  We should only examine the dirty
> memory thresholds and throttle once per "batch", not once per page.

That should do it 

--- mm/vmscan.c.orig	2004-08-30 20:19:05.000000000 -0300
+++ mm/vmscan.c	2004-08-31 08:30:08.323989416 -0300
@@ -73,6 +73,10 @@
 	unsigned int gfp_mask;
 
 	int may_writepage;
+
+	int inflight;
+
+	int throttled; /* how many times have we throttled on VM inflight IO limit */
 };
 
 /*
@@ -245,8 +249,30 @@
 	return page_count(page) - !!PagePrivate(page) == 2;
 }
 
-static int may_write_to_queue(struct backing_dev_info *bdi)
+/*
+ * This function calculates the maximum pinned-for-IO memory
+ * the page eviction threads can generate. If we hit the max,
+ * we throttle taking a nap.
+ *
+ * Returns true if we cant writeout.
+ */
+int vm_eviction_limits(struct scan_control *sc)
+{
+        if (sc->inflight > (totalram_pages * vm_dirty_ratio) / 100)  {
+		if (sc->throttled < 5) {
+			blk_congestion_wait(WRITE, HZ/5);
+			sc->throttled++;
+		}
+                return 1;
+        }
+        return 0;
+}
+
+static int may_write_to_queue(struct backing_dev_info *bdi, struct scan_control *sc)
 {
+	if (vm_eviction_limits(sc)) /* Check VM writeout limit */
+		return 0;
+
 	if (current_is_kswapd())
 		return 1;
 	if (current_is_pdflush())	/* This is unlikely, but why not... */
@@ -286,7 +312,7 @@
 /*
  * pageout is called by shrink_list() for each dirty page. Calls ->writepage().
  */
-static pageout_t pageout(struct page *page, struct address_space *mapping)
+static pageout_t pageout(struct page *page, struct address_space *mapping, struct scan_control *sc)
 {
 	/*
 	 * If the page is dirty, only perform writeback if that write
@@ -311,7 +337,7 @@
 		return PAGE_KEEP;
 	if (mapping->a_ops->writepage == NULL)
 		return PAGE_ACTIVATE;
-	if (!may_write_to_queue(mapping->backing_dev_info))
+	if (!may_write_to_queue(mapping->backing_dev_info, sc))
 		return PAGE_KEEP;
 
 	if (clear_page_dirty_for_io(page)) {
@@ -421,7 +447,7 @@
 				goto keep_locked;
 
 			/* Page is dirty, try to write it out here */
-			switch(pageout(page, mapping)) {
+			switch(pageout(page, mapping, sc)) {
 			case PAGE_KEEP:
 				goto keep_locked;
 			case PAGE_ACTIVATE:
@@ -807,6 +833,7 @@
 		nr_inactive = 0;
 
 	sc->nr_to_reclaim = SWAP_CLUSTER_MAX;
+	sc->throttled = 0;
 
 	while (nr_active || nr_inactive) {
 		if (nr_active) {
@@ -819,6 +846,7 @@
 		if (nr_inactive) {
 			sc->nr_to_scan = min(nr_inactive,
 					(unsigned long)SWAP_CLUSTER_MAX);
+			sc->inflight = read_page_state(nr_writeback);
 			nr_inactive -= sc->nr_to_scan;
 			shrink_cache(zone, sc);
 			if (sc->nr_to_reclaim <= 0)
--
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