Re: swapping and the value of /proc/sys/vm/swappiness

[Ray Bryant <raybry@sgi.com>]

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Nick,

As reported to you elsewhere (and duplicated here to get on this thread),
application of the patch you sent (attached) dramatically changes the
swappiness behavior of the 2.6.9-rc1 (and presumably the rc2) kernel.

Here are the updated results:

Previously:

Kernel Version 2.6.9-rc1-mm3:
         Total I/O   Avg Swap   min    max     pg cache    min    max
        ----------- --------- ------- ------  --------- ------- -------
    0   274.80 MB/s  10511 MB (  5644, 14492)  13293 MB (  8596, 17156)
   20   267.02 MB/s  12624 MB (  5578, 16287)  15298 MB (  8468, 18889)
   40   267.66 MB/s  13541 MB (  6619, 17461)  16199 MB (  9393, 20044)
   60   233.73 MB/s  18094 MB ( 16550, 19676)  20629 MB ( 19103, 22192)
   80   213.64 MB/s  20950 MB ( 15844, 22977)  23450 MB ( 18496, 25440)
  100   164.58 MB/s  26004 MB ( 26004, 26004)  28410 MB ( 28327, 28455)

With Nick Piggin et al fix:

Kernel Version: linux-2.6.9-rc1-mm3-kswapdfix

         Total I/O   Avg Swap   min    max     pg cache    min    max
        ----------- --------- ------- ------  --------- ------- -------
    0   279.97 MB/s     89 MB (    12,   265)   3062 MB (  2947,  3267)
   20   283.55 MB/s    161 MB (    15,   372)   3190 MB (  3011,  3427)
   40   282.32 MB/s    204 MB (     6,   407)   3187 MB (  2995,  3331)
   60   279.42 MB/s     72 MB (    15,   171)   3091 MB (  3027,  3155)
   80   283.34 MB/s    920 MB (   144,  3028)   3904 MB (  3106,  5957)
  100   160.55 MB/s  26008 MB ( 26007, 26008)  28473 MB ( 28455, 28487)

(The drop at swappiness of 60 may just be randomness, not sure it
is significant, but these results are all based on 5 trials.)

At any rate, this patch appears to fix the problems I was seeing before.
(See
	http://marc.theaimsgroup.com/?l=linux-kernel&m=109449778320333&w=2

for further details of the benchmark and the test environment).
-- 
Best Regards,
Ray
-----------------------------------------------
                   Ray Bryant
512-453-9679 (work)         512-507-7807 (cell)
raybry@sgi.com             raybry@austin.rr.com
The box said: "Requires Windows 98 or better",
            so I installed Linux.
-----------------------------------------------

[-- Attachment #2: vm-no-wild-kswapd.patch --]
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---

 linux-2.6-npiggin/mm/vmscan.c |   14 +++++++++++++-
 1 files changed, 13 insertions(+), 1 deletion(-)

diff -puN mm/vmscan.c~vm-no-wild-kswapd mm/vmscan.c
--- linux-2.6/mm/vmscan.c~vm-no-wild-kswapd	2004-09-25 10:09:16.000000000 +1000
+++ linux-2.6-npiggin/mm/vmscan.c	2004-09-25 10:15:58.000000000 +1000
@@ -993,10 +993,13 @@ static int balance_pgdat(pg_data_t *pgda
 	int to_free = nr_pages;
 	int priority;
 	int i;
-	int total_scanned = 0, total_reclaimed = 0;
+	int total_scanned, total_reclaimed;
 	struct reclaim_state *reclaim_state = current->reclaim_state;
 	struct scan_control sc;
 
+loop_again:
+	total_scanned = 0;
+	total_reclaimed = 0;
 	sc.gfp_mask = GFP_KERNEL;
 	sc.may_writepage = 0;
 	sc.nr_mapped = read_page_state(nr_mapped);
@@ -1095,6 +1098,15 @@ scan:
 		 */
 		if (total_scanned && priority < DEF_PRIORITY - 2)
 			blk_congestion_wait(WRITE, HZ/10);
+
+		/*
+		 * We do this so kswapd doesn't build up large priorities for
+		 * example when it is freeing in parallel with allocators. It
+		 * matches the direct reclaim path behaviour in terms of impact
+		 * on zone->*_priority.
+		 */
+		if (total_reclaimed >= 32)
+			goto loop_again;
 	}
 out:
 	for (i = 0; i < pgdat->nr_zones; i++) {

_