Per-mount syncer threads and fanout for pagedaemon cleaning

[Attilio Rao]

2011/12/27 Giovanni Trematerra <giovanni.trematerra at gmail.com>:
> On Mon, Dec 26, 2011 at 9:24 PM, Venkatesh Srinivas
> <vsrinivas at dragonflybsd.org> wrote:
>> Hi!
>>
>> I've been playing with two things in DragonFly that might be of interest
>> here.
>>
>> Thing #1 :=
>>
>> First, per-mountpoint syncer threads. Currently there is a single thread,
>> 'syncer', which periodically calls fsync() on dirty vnodes from every mount,
>> along with calling vfs_sync() on each filesystem itself (via syncer vnodes).
>>
>> My patch modifies this to create syncer threads for mounts that request it.
>> For these mounts, vnodes are synced from their mount-specific thread rather
>> than the global syncer.
>>
>> The idea is that periodic fsync/sync operations from one filesystem should
>> not
>> stall or delay synchronization for other ones.
>> The patch was fairly simple:
>> http://gitweb.dragonflybsd.org/dragonfly.git/commitdiff/50e4012a4b55e1efc595db0db397b4365f08b640
>>
>
> There's something WIP by attilio@ on that area.
> you might want to take a look at
> http://people.freebsd.org/~attilio/syncer_alpha_15.diff
>
> I don't know what hammerfs needs but UFS/FFS and buffer cache make a good
> job performance-wise and so the authors are skeptical about the boost that such
> a change can give. We believe that brain cycles need to be spent on
> other pieces of the system such as ARC and ZFS.

More specifically, it is likely that focusing on UFS and buffer cache
for performance is not really useful, we should drive our efforts over
ARC and ZFS.
Also, the real bottlenecks in our I/O paths are in GEOM
single-threaded design, lack of unmapped I/O functionality, possibly
lack of proritized I/O, etc.

Attilio


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