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

Mateusz Guzik
2024-06-29T15:46:41Z

How about you post a working version of the patchset, even if it only
looks at 0-63 and we are going to massage it afterwards.

On Sat, Jun 29, 2024 at 5:41 PM Ma, Yu <yu.ma@intel.com> wrote:
>
>
> On 6/28/2024 5:12 PM, Jan Kara wrote:
> > On Thu 27-06-24 21:59:12, Mateusz Guzik wrote:
> >> On Thu, Jun 27, 2024 at 8:27 PM Ma, Yu <yu.ma@intel.com> wrote:
> >>> 2. For fast path implementation, the essential and simple point is to
> >>> directly return an available bit if there is free bit in [0-63]. I'd
> >>> emphasize that it does not only improve low number of open fds (even it
> >>> is the majority case on system as Honza agreed), but also improve the
> >>> cases that lots of fds open/close frequently with short task (as per the
> >>> algorithm, lower bits will be prioritized to allocate after being
> >>> recycled). Not only blogbench, a synthetic benchmark, but also the
> >>> realistic scenario as claimed in f3f86e33dc3d("vfs: Fix pathological
> >>> performance case for __alloc_fd()"), which literally introduced this
> >>> 2-levels bitmap searching algorithm to vfs as we see now.
> >> I don't understand how using next_fd instead is supposed to be inferior.
> >>
> >> Maybe I should clarify that by API contract the kernel must return the
> >> lowest free fd it can find. To that end it maintains the next_fd field
> >> as a hint to hopefully avoid some of the search work.
> >>
> >> In the stock kernel the first thing done in alloc_fd is setting it as
> >> a starting point:
> >> fdt = files_fdtable(files);
> >> fd = start;
> >> if (fd < files->next_fd)
> >> fd = files->next_fd;
> >>
> >> that is all the calls which come here with 0 start their search from
> >> next_fd position.
> > Yup.
> >
> >> Suppose you implemented the patch as suggested by me and next_fd fits
> >> the range of 0-63. Then you get the benefit of lower level bitmap
> >> check just like in the patch you submitted, but without having to
> >> first branch on whether you happen to be in that range.
> >>
> >> Suppose next_fd is somewhere higher up, say 80. With your general
> >> approach the optimization wont be done whatsoever or it will be
> >> attempted at the 0-63 range when it is an invariant it finds no free
> >> fds.
> >>
> >> With what I'm suggesting the general idea of taking a peek at the
> >> lower level bitmap can be applied across the entire fd space. Some
> >> manual mucking will be needed to make sure this never pulls more than
> >> one cacheline, easiest way out I see would be to align next_fd to
> >> BITS_PER_LONG for the bitmap search purposes.
> > Well, all you need to do is to call:
> >
> > bit = find_next_zero_bit(fdt->open_fds[start / BITS_PER_LONG],
> > BITS_PER_LONG, start & (BITS_PER_LONG-1));
> > if (bit < BITS_PER_LONG)
> > return bit + (start & ~(BITS_PER_LONG - 1));
> >
> >
> > in find_next_fd(). Not sure if this is what you meant by aligning next_fd
> > to BITS_PER_LONG...
> >
> > Honza
>
> So the idea here is to take a peek at the word contains next_fd, return
> directly if get free bit there. Not sure about the efficiency here if
> open/close fd frequently and next_fd is distributed very randomly. Just
> give a quick try for this part of code, seems kernel failed to boot
> up😳, kind of out of expectation ...
>

--
Mateusz Guzik <mjguzik gmail.com>