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

Ma, Yu
2024-07-09T08:32:08Z

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;
>>> +       }
>> 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