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

[Ma, Yu]

On 7/5/2024 3:56 PM, Ma, Yu wrote:
> 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
Hi Guzik, Honza,

Do we have any more comment or idea regarding to the fast path? Thanks 
for your time and any feedback :)


Regards

Yu