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

Ma, Yu
2024-06-19T17:09:57Z

On 6/19/2024 6:36 PM, David Laight wrote:
> From: Yu Ma <yu.ma@intel.com>
>> Sent: 14 June 2024 17:34
>>
>> There is available fd in the lower 64 bits of open_fds bitmap for most cases
>> when we look for an available fd slot. Skip 2-levels searching via
>> find_next_zero_bit() for this common fast path.
>>
>> Look directly for an open bit in the lower 64 bits of open_fds bitmap when a
>> free slot is available there, as:
>> (1) The fd allocation algorithm would always allocate fd from small to large.
>> Lower bits in open_fds bitmap would be used much more frequently than higher
>> bits.
>> (2) After fdt is expanded (the bitmap size doubled for each time of expansion),
>> it would never be shrunk. The search size increases but there are few open fds
>> available here.
>> (3) There is fast path inside of find_next_zero_bit() when size<=64 to speed up
>> searching.
>>
>> With the fast path added in alloc_fd() through one-time bitmap searching,
>> pts/blogbench-1.1.0 read is improved by 20% and write by 10% on Intel ICX 160
>> cores configuration with v6.8-rc6.
>>
>> Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
>> Signed-off-by: Yu Ma <yu.ma@intel.com>
>> ---
>> fs/file.c | 9 +++++++--
>> 1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/file.c b/fs/file.c
>> index 3b683b9101d8..e8d2f9ef7fd1 100644
>> --- a/fs/file.c
>> +++ b/fs/file.c
>> @@ -510,8 +510,13 @@ static int alloc_fd(unsigned start, unsigned end, unsigned flags)
>> if (fd < files->next_fd)
>> fd = files->next_fd;
>>
>> - if (fd < fdt->max_fds)
>> + if (fd < fdt->max_fds) {
>> + if (~fdt->open_fds[0]) {
>> + fd = find_next_zero_bit(fdt->open_fds, BITS_PER_LONG, fd);
>> + goto success;
>> + }
>> fd = find_next_fd(fdt, fd);
>> + }
> Hmm...
> How well does that work when the initial fd is > 64?
>
> Since there is exactly one call to find_next_fd() and it is static and should
> be inlined doesn't this optimisation belong inside find_next_fd().
>
> Plausibly find_next_fd() just needs rewriting.
The consideration for this fast path is as stated in commit, for
scenarios like fd>64, it means that fast path already worked in the
first 64 bits for fast return and all other times when any fd<64 gets
recycled and then allocated. For some cases like a process opened more
than 64 fds and kept occupied, the extra cost would be a conditional
statement which can be benefit from branch prediction, as Guzik
suggests, we'll copy Eric for benchmark to check the effect if it is
available.  For the code, it's more efficient to be here outside of
find_next_fd() for jumping to fast return. Besides, identified by Guzik,
find_next_fd() itself could be improved with inlined calls inside for
better performance, story for another patch :)
>
> Or, possibly. even inside an inlinable copy of find_next_zero-bit()
> (although a lot of callers won't be 'hot' enough for the inlined bloat
> being worth while).
As mentioned, current find_next_zero_bit() already has a fast path
inside to handle the searching size <= 64, and it has been utilized here
for fast return.
>
> David
> -
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