Re: [RFC PATCH linux 2/2] fs/proc: use a hash table for the directory entries

[Nicolas Dichtel]

Le 02/10/2014 20:01, Eric W. Biederman a Ãcrit :
> Nicolas Dichtel <nicolas.dichtel@xxxxxxxxx> writes:
>
> > From: Thierry Herbelot <thierry.herbelot@xxxxxxxxx>
> >
> > The current implementation for the directories in /proc is using a single
> > linked list. This is slow when handling directories with large numbers of
> > entries (eg netdevice-related entries when lots of tunnels are opened).
> >
> > This patch enables multiple linked lists. A hash based on the entry name is
> > used to select the linked list for one given entry.
> >
> > The speed creation of netdevices is faster as shorter linked lists must be
> > scanned when adding a new netdevice.
>
> Is the directory of primary concern /proc/net/dev/snmp6 ?
Yes.

> Unless I have configured my networking stack weird by mistake that
> is the only directory under /proc/net that grows when we add an
> interface.
>
> I just want to make certain I am seeing the same things that you are
> seeing.
>
> I feel silly for overlooking this directory when the rest of the
> scalability work was done.
>
> > Here are some numbers:
> >
> > dummy30000.batch contains 30 000 times 'link add type dummy'.
> >
> > Before the patch:
> > time ip -b dummy30000.batch
> > real    2m32.221s
> > user    0m0.380s
> > sys     2m30.610s
> >
> > After the patch:
> > time ip -b dummy30000.batch
> > real    1m57.190s
> > user    0m0.350s
> > sys     1m56.120s
> >
> > The single 'subdir' list head is replaced by a subdir hash table. The subdir
> > hash buckets are only allocated for directories. The number of hash buckets
> > is a compile-time parameter.
>
> That looks like a nice speed up.  A couple of things.
>
> With sysfs and sysctl when faced this class of challenge we used an
> rbtree instead of a hash table.  That should use less memory and scale
> better.
>
> I am concerned about a fixed sized hash table moving the location where
> we fall off a cliff but not removing the cliff itself.
>
> I suppose it would be possible to use the new fancy resizable hash
> tables but previous work on sysctl and sysfs suggests that we don't look
> up these entries sufficiently to require a hash table.  We just need a
> data structure that doesn't fall over at scale, and the rbtrees seem to
> do that very nicely.
Ok, I will have a look at it.



Thank you,
Nicolas
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