Re: [PATCH v2 1/3] fs/file.c: add fast path in alloc_fd()

Mateusz Guzik
2024-06-26T19:13:22Z

On Wed, Jun 26, 2024 at 1:54 PM Jan Kara <jack@suse.cz> wrote:
> So maybe I'm wrong but I think the biggest benefit of your code compared to
> plain find_next_fd() is exactly in that we don't have to load full_fds_bits
> into cache. So I'm afraid that using full_fds_bits in the condition would
> destroy your performance gains. Thinking about this with a fresh head how
> about putting implementing your optimization like:
>
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -490,6 +490,20 @@ static unsigned int find_next_fd(struct fdtable *fdt, unsigned int start)
> unsigned int maxbit = maxfd / BITS_PER_LONG;
> unsigned int bitbit = start / BITS_PER_LONG;
>
> + /*
> + * Optimistically search the first long of the open_fds bitmap. It
> + * saves us from loading full_fds_bits into cache in the common case
> + * and because BITS_PER_LONG > start >= files->next_fd, we have quite
> + * a good chance there's a bit free in there.
> + */
> + if (start < BITS_PER_LONG) {
> + unsigned int bit;
> +
> + bit = find_next_zero_bit(fdt->open_fds, BITS_PER_LONG, start);
> + if (bit < BITS_PER_LONG)
> + return bit;
> + }
> +
> bitbit = find_next_zero_bit(fdt->full_fds_bits, maxbit, bitbit) * BITS_PER_LONG;
> if (bitbit >= maxfd)
> return maxfd;
>
> Plus your optimizations with likely / unlikely. This way the code flow in
> alloc_fd() stays more readable, we avoid loading the first open_fds long
> into cache if it is full, and we should get all the performance benefits?
>

Huh.

So when I read the patch previously I assumed this is testing the bit
word for the map containing next_fd (whatever it is), avoiding looking
at the higher level bitmap and inlining the op (instead of calling the
fully fledged func for bit scans).

I did not mentally register this is in fact only checking for the
beginning of the range of the entire thing. So apologies from my end
as based on my feedback some work was done and I'm going to ask to
further redo it.

blogbench spawns 100 or so workers, say total fd count hovers just
above 100. say this lines up with about half of more cases in practice
for that benchmark.

Even so, that's a benchmark-specific optimization. A busy web server
can have literally tens of thousands of fds open (and this is a pretty
mundane case), making the 0-63 range not particularly interesting.

That aside I think the patchset is in the wrong order -- first patch
tries to not look at the higher level bitmap, while second reduces
stores made there. This makes it quite unclear how much is it worth to
reduce looking there if atomics are conditional.

So here is what I propose in terms of the patches:
1. NULL check removal, sprinkling of likely/unlikely and expand_files
call avoidance; no measurements done vs stock kernel for some effort
saving, just denote in the commit message there is less work under the
lock and treat it as baseline
2. conditional higher level bitmap clear as submitted; benchmarked against 1
3. open_fds check within the range containing fd, avoiding higher
level bitmap if a free slot is found. this should not result in any
func calls if successful; benchmarked against the above

Optionally the bitmap routines can grow variants which always inline
and are used here. If so that would probably land between 1 and 2 on
the list.

You noted you know about blogbench bugs and have them fixed. Would be
good to post a link to a pull request or some other spot for a
reference.

I'll be best if the vfs folk comment on what they want here.
--
Mateusz Guzik <mjguzik gmail.com>