Re: [PATCH v6 4/5] 9pfs: T_readdir latency optimization

[Greg Kurz]

On Wed, 03 Jun 2020 19:16:08 +0200
Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:

> On Sonntag, 19. April 2020 17:06:17 CEST Christian Schoenebeck wrote:
> > Make top half really top half and bottom half really bottom half:
> > 
> > Each T_readdir request handling is hopping between threads (main
> > I/O thread and background I/O driver threads) several times for
> > every individual directory entry, which sums up to huge latencies
> > for handling just a single T_readdir request.
> > 
> > Instead of doing that, collect now all required directory entries
> > (including all potentially required stat buffers for each entry) in
> > one rush on a background I/O thread from fs driver by calling the
> > previously added function v9fs_co_readdir_many() instead of
> > v9fs_co_readdir(), then assemble the entire resulting network
> > response message for the readdir request on main I/O thread. The
> > fs driver is still aborting the directory entry retrieval loop
> > (on the background I/O thread inside of v9fs_co_readdir_many())
> > as soon as it would exceed the client's requested maximum R_readdir
> > response size. So this will not introduce a performance penalty on
> > another end.
> > 
> > Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> > ---
> >  hw/9pfs/9p.c | 122 +++++++++++++++++++++++----------------------------
> >  1 file changed, 55 insertions(+), 67 deletions(-)
> 
> Ping. Anybody?
> 
> I would like to roll out this patch set soon and this is the only patch in 
> this series missing a review yet.
> 

Hi Christian,

Sorry for getting back to this only now :-\

So I still have some concerns about the locking of the directory stream
pointer a fid. It was initially introduced to avoid concurrent accesses
by multiple threads to the corresponding internal glibc object, as
recommended in the readdir(3) manual page. Now, this patch considerably
extends the critical section to also contain calls to telldir() and all
the _many_ readdir()... so I'm not sure exactly what's the purpose of
that mutex right now. Please provide more details.

Cheers,

--
Greg