ping...
On Tue, Feb 18, 2014 at 04:01:22PM +0800, Yuanhan Liu wrote:
> 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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