On 02/18/2014 04:01 PM, 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).
>
>From my understanding, I also think we used to reclaim more memory if
sc->priority==DEF_PRIORITY. See the while loop:
while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
nr[LRU_INACTIVE_FILE]) {
For kswapd, the loop will continue until nr[LRU_INACTIVE_ANON],
nr[LRU_ACTIVE_FILE] and nr[LRU_INACTIVE_FILE] become zero.
But in commit e82e0561("mm: vmscan: obey proportional scanning
requirements for kswapd"), nr[lru] was set to 0.
/* Stop scanning the smaller of the LRU */
nr[lru] = 0;
nr[lru + LRU_ACTIVE] = 0;
And the other LRU scan count was also recalculated, as a result the
total scan count in this round may less than original code.
So I think this change is reasonable which make the behaviour the same
as before(also no performance drop).
--
Regards,
-Bob
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