On 12/11/20 11:32 PM, Konstantin Khlebnikov wrote:
> On Thu, Dec 10, 2020 at 2:01 AM Junxiao Bi <junxiao.bi@oracle.com
> <mailto:junxiao.bi@oracle.com>> wrote:
>
> Hi Konstantin,
>
> We tested this patch set recently and found it limiting negative
> dentry
> to a small part of total memory. And also we don't see any
> performance
> regression on it. Do you have any plan to integrate it into
> mainline? It
> will help a lot on memory fragmentation issue causing by dentry slab,
> there were a lot of customer cases where sys% was very high since
> most
> cpu were doing memory compaction, dentry slab was taking too much
> memory
> and nearly all dentry there were negative.
>
>
> Right now I don't have any plans for this. I suspect such problems will
> appear much more often since machines are getting bigger.
> So, somebody will take care of it.
We already had a lot of customer cases. It made no sense to leave so
many negative dentry in the system, it caused memory fragmentation and
not much benefit.
Dcache could grow so big only if the system lacks of memory pressure.
Simplest solution is a cronjob which provinces such pressure by
creating sparse file on disk-based fs and then reading it.
This should wash away all inactive caches with no IO and zero chance of oom.
>
> First part which collects negative dentries at the end list of
> siblings could be
> done in a more obvious way by splitting the list in two.
> But this touches much more code.
That would add new field to dentry?
Yep. Decision is up to maintainers.
>
> Last patch isn't very rigid but does non-trivial changes.
> Probably it's better to call some garbage collector thingy periodically.
> Lru list needs pressure to age and reorder entries properly.
Swap the negative dentry to the head of hash list when it get accessed?
Extra ones can be easily trimmed when swapping, using GC is to reduce
perf impact?
Reclaimer/shrinker scans denties in LRU lists, it's an another list.
My patch used order in hash lists is a very unusual way. Don't be confused.
There are four lists
parent - siblings
hashtable - hashchain
LRU
inode - alias
Thanks,
Junxioao.
>
> Gc could be off by default or thresholds set very high (50% of ram for
> example).
> Final setup could be left up to owners of large systems, which needs
> fine tuning.