Hi,
Commit e82e0561("mm: vmscan: obey proportional scanning requirements for
kswapd") caused a big performance regression(73%) for vm-scalability/
lru-file-readonce testcase on a system with 256G memory without swap.
That testcase simply looks like this:
truncate -s 1T /tmp/vm-scalability.img
mkfs.xfs -q /tmp/vm-scalability.img
mount -o loop /tmp/vm-scalability.img /tmp/vm-scalability
SPARESE_FILE="/tmp/vm-scalability/sparse-lru-file-readonce"
for i in `seq 1 120`; do
truncate $SPARESE_FILE-$i -s 36G
timeout --foreground -s INT 300 dd bs=4k if=$SPARESE_FILE-$i of=/dev/null
done
wait
Actually, it's not the newlly added code(obey proportional scanning)
in that commit caused the regression. But instead, it's the following
change:
+
+ if (nr_reclaimed < nr_to_reclaim || scan_adjusted)
+ continue;
+
- if (nr_reclaimed >= nr_to_reclaim &&
- sc->priority < DEF_PRIORITY)
+ if (global_reclaim(sc) && !current_is_kswapd())
break;
The difference is that we might reclaim more than requested before
in the first round reclaimming(sc->priority == DEF_PRIORITY).
So, for a testcase like lru-file-readonce, the dirty rate is fast, and
reclaimming SWAP_CLUSTER_MAX(32 pages) each time is not enough for catching
up the dirty rate. And thus page allocation stalls, and performance drops:
O for e82e0561
* for parent commit
proc-vmstat.allocstall
2e+06 ++---------------------------------------------------------------+
1.8e+06 O+ O O O |
| |
1.6e+06 ++ |
1.4e+06 ++ |
| |
1.2e+06 ++ |
1e+06 ++ |
800000 ++ |
| |
600000 ++ |
400000 ++ |
| |
200000 *+..............*................*...............*...............*
0 ++---------------------------------------------------------------+
vm-scalability.throughput
2.2e+07 ++---------------------------------------------------------------+
| |
2e+07 *+..............*................*...............*...............*
1.8e+07 ++ |
| |
1.6e+07 ++ |
| |
1.4e+07 ++ |
| |
1.2e+07 ++ |
1e+07 ++ |
| |
8e+06 ++ O O O |
O |
6e+06 ++---------------------------------------------------------------+
I made a patch which simply keeps reclaimming more if sc->priority == DEF_PRIORITY.
I'm not sure it's the right way to go or not. Anyway, I pasted it here for comments.
---
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 26ad67f..37004a8 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1828,7 +1828,16 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
unsigned long nr_reclaimed = 0;
unsigned long nr_to_reclaim = sc->nr_to_reclaim;
struct blk_plug plug;
- bool scan_adjusted = false;
+ /*
+ * On large memory systems, direct reclamming of SWAP_CLUSTER_MAX
+ * each time may not catch up the dirty rate in some cases(say,
+ * vm-scalability/lru-file-readonce), which may increase the
+ * page allocation stall latency in the end.
+ *
+ * Here we try to reclaim more than requested for the first round
+ * (sc->priority == DEF_PRIORITY) to reduce such latency.
+ */
+ bool scan_adjusted = sc->priority == DEF_PRIORITY;
get_scan_count(lruvec, sc, nr);
--
1.7.7.6
--yliu
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