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

Ma, Yu
2024-07-11T09:29:21Z

On 7/11/2024 7:40 AM, Tim Chen wrote:
> On Tue, 2024-07-09 at 12:17 +0200, Mateusz Guzik wrote:
>> Right, forgot to respond.
>>
>> I suspect the different result is either because of mere variance
>> between reboots or blogbench using significantly less than 100 fds at
>> any given time -- I don't have an easy way to test at your scale at
>> the moment. You could probably test that by benching both approaches
>> while switching them at runtime with a static_branch. However, I don't
>> know if that effort is warranted atm.
>>
>> So happens I'm busy with other stuff and it is not my call to either
>> block or let this in, so I'm buggering off.
>>
>> On Tue, Jul 9, 2024 at 10:32 AM Ma, Yu <yu.ma@intel.com> wrote:
>>>
>>> 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;
>>>>>> + }
> I think this approach based on next_fd quick check is more generic and scalable.
>
> It just happen for blogbench, just checking the first 64 bit allow a quicker
> skip to the two level search where this approach, next_fd may be left
> in a 64 word that actually has no open bits and we are doing useless search
> in find_next_zero_bit(). Perhaps we should check full_fds_bits to make sure
> there are empty slots before we do
> find_next_zero_bit() fast path. Something like
>
> if (!test_bit(bitbit, fdt->full_fds_bits)) {
> 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;
> }
> Tim

Yes, agree that it scales better, I'll update v4 with fast path for the
word contains next_fd and send out for review soon

>>>>> 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
>>>
>>