Re: [PATCH v3 3/3] fs/file.c: add fast path in find_next_fd()

[Ma, Yu]

On 7/5/2024 5:55 AM, Jan Kara wrote:
> On Thu 04-07-24 19:44:10, Mateusz Guzik wrote:
> > On Wed, Jul 3, 2024 at 4:07 PM Yu Ma <yu.ma@xxxxxxxxx> wrote:
> > > There is available fd in the lower 64 bits of open_fds bitmap for most cases
> > > when we look for an available fd slot. Skip 2-levels searching via
> > > find_next_zero_bit() for this common fast path.
> > >
> > > Look directly for an open bit in the lower 64 bits of open_fds bitmap when a
> > > free slot is available there, as:
> > > (1) The fd allocation algorithm would always allocate fd from small to large.
> > > Lower bits in open_fds bitmap would be used much more frequently than higher
> > > bits.
> > > (2) After fdt is expanded (the bitmap size doubled for each time of expansion),
> > > it would never be shrunk. The search size increases but there are few open fds
> > > available here.
> > > (3) There is fast path inside of find_next_zero_bit() when size<=64 to speed up
> > > searching.
> > >
> > > As suggested by Mateusz Guzik <mjguzik gmail.com> and Jan Kara <jack@xxxxxxx>,
> > > update the fast path from alloc_fd() to find_next_fd(). With which, on top of
> > > patch 1 and 2, pts/blogbench-1.1.0 read is improved by 13% and write by 7% on
> > > Intel ICX 160 cores configuration with v6.10-rc6.
> > >
> > > Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > > Signed-off-by: Yu Ma <yu.ma@xxxxxxxxx>
> > > ---
> > >   fs/file.c | 5 +++++
> > >   1 file changed, 5 insertions(+)
> > >
> > > diff --git a/fs/file.c b/fs/file.c
> > > index a15317db3119..f25eca311f51 100644
> > > --- a/fs/file.c
> > > +++ b/fs/file.c
> > > @@ -488,6 +488,11 @@ struct files_struct init_files = {
> > >
> > >   static unsigned int find_next_fd(struct fdtable *fdt, unsigned int start)
> > >   {
> > > +       unsigned int bit;
> > > +       bit = find_next_zero_bit(fdt->open_fds, BITS_PER_LONG, start);
> > > +       if (bit < BITS_PER_LONG)
> > > +               return bit;
> > > +
> > >          unsigned int maxfd = fdt->max_fds; /* always multiple of BITS_PER_LONG */
> > >          unsigned int maxbit = maxfd / BITS_PER_LONG;
> > >          unsigned int bitbit = start / BITS_PER_LONG;
> > > --
> > > 2.43.0
> > I had something like this in mind:
> > diff --git a/fs/file.c b/fs/file.c
> > index a3b72aa64f11..4d3307e39db7 100644
> > --- a/fs/file.c
> > +++ b/fs/file.c
> > @@ -489,6 +489,16 @@ static unsigned int find_next_fd(struct fdtable
> > *fdt, unsigned int start)
> >          unsigned int maxfd = fdt->max_fds; /* always multiple of
> > BITS_PER_LONG */
> >          unsigned int maxbit = maxfd / BITS_PER_LONG;
> >          unsigned int bitbit = start / BITS_PER_LONG;
> > +       unsigned int bit;
> > +
> > +       /*
> > +        * Try to avoid looking at the second level map.
> > +        */
> > +       bit = find_next_zero_bit(&fdt->open_fds[bitbit], BITS_PER_LONG,
> > +                               start & (BITS_PER_LONG - 1));
> > +       if (bit < BITS_PER_LONG) {
> > +               return bit + bitbit * BITS_PER_LONG;
> > +       }
> Drat, you're right. I missed that Ma did not add the proper offset to
> open_fds. *This* is what I meant :)
>
> 								Honza

Just tried this on v6.10-rc6, the improvement on top of patch 1 and 
patch 2 is 7% for read and 3% for write, less than just check first word.

Per my understanding, its performance would be better if we can find 
free bit in the same word of next_fd with high possibility, but next_fd 
just represents the lowest possible free bit. If fds are open/close 
frequently and randomly, that might not always be the case, next_fd may 
be distributed randomly, for example, 0-65 are occupied, fd=3 is 
returned, next_fd will be set to 3, next time when 3 is allocated, 
next_fd will be set to 4, while the actual first free bit is 66 , when 
66 is allocated, and fd=5 is returned, then the above process would be 
went through again.

Yu