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

Ma, Yu
2024-07-05T07:56:57Z

On 7/5/2024 5:55 AM, Jan Kara wrote:
> On Thu 04-07-24 19:44:10, Mateusz Guzik wrote:
>> On Wed, Jul 3, 2024 at 4:07 PM Yu Ma <yu.ma@intel.com> wrote:
>>> 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.
>>>
>>> As suggested by Mateusz Guzik <mjguzik gmail.com> and Jan Kara <jack@suse.cz>,
>>> update the fast path from alloc_fd() to find_next_fd(). With which, on top of
>>> patch 1 and 2, pts/blogbench-1.1.0 read is improved by 13% and write by 7% on
>>> Intel ICX 160 cores configuration with v6.10-rc6.
>>>
>>> Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
>>> Signed-off-by: Yu Ma <yu.ma@intel.com>
>>> ---
>>> fs/file.c | 5 +++++
>>> 1 file changed, 5 insertions(+)
>>>
>>> diff --git a/fs/file.c b/fs/file.c
>>> index a15317db3119..f25eca311f51 100644
>>> --- a/fs/file.c
>>> +++ b/fs/file.c
>>> @@ -488,6 +488,11 @@ struct files_struct init_files = {
>>>
>>> static unsigned int find_next_fd(struct fdtable *fdt, unsigned int start)
>>> {
>>> + unsigned int bit;
>>> + bit = find_next_zero_bit(fdt->open_fds, BITS_PER_LONG, start);
>>> + if (bit < BITS_PER_LONG)
>>> + return bit;
>>> +
>>> 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;
>>> --
>>> 2.43.0
>>>
>> 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