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

[Tim Chen]

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

> > > > 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
> > 
> 
>