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

[Ma, Yu]

On 7/11/2024 7:40 AM, Tim Chen wrote:
> On Tue, 2024-07-09 at 12:17 +0200, Mateusz Guzik wrote:
> > Right, forgot to respond.
> >
> > I suspect the different result is either because of mere variance
> > between reboots or blogbench using significantly less than 100 fds at
> > any given time -- I don't have an easy way to test at your scale at
> > the moment. You could probably test that by benching both approaches
> > while switching them at runtime with a static_branch. However, I don't
> > know if that effort is warranted atm.
> >
> > So happens I'm busy with other stuff and it is not my call to either
> > block or let this in, so I'm buggering off.
> >
> > On Tue, Jul 9, 2024 at 10:32 AM Ma, Yu <yu.ma@xxxxxxxxx> wrote:
> > > 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;
> > > > > > +       }
> I think this approach based on next_fd quick check is more generic and scalable.
>
> It just happen for blogbench, just checking the first 64 bit allow a quicker
> skip to the two level search where this approach, next_fd may be left
> in a 64 word that actually has no open bits and we are doing useless search
> in find_next_zero_bit(). Perhaps we should check full_fds_bits to make sure
> there are empty slots before we do
> find_next_zero_bit() fast path.  Something like
>
> 	if (!test_bit(bitbit, fdt->full_fds_bits)) {
> 		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;
> 	}
> Tim

Yes, agree that it scales better, I'll update v4 with fast path for the 
word contains next_fd and send out for review soon

> > > > > 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