Re: [PATCH RFC 0/8] dcache: increase poison resistance

[Junxiao Bi <junxiao.bi@oracle.com>]

On 12/13/20 11:43 PM, Konstantin Khlebnikov wrote:

>
>
> On Sun, Dec 13, 2020 at 9:52 PM Junxiao Bi <junxiao.bi@oracle.com 
> <mailto:junxiao.bi@oracle.com>> wrote:
>
>     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>
>     > <mailto: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.
Sound good, will try.
>
>     >
>     > 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.

Ah, you mean GC to reclaim from LRU list. I am not sure it could catch 
up the speed of negative dentry generating.

Thanks,

Junxiao.

> 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.
>