On 5/6/21 12:55 PM, Al Viro wrote:
> On Thu, May 06, 2021 at 11:19:03AM -0600, Jens Axboe wrote:
> 
>> Doing a quick profile, on the latter run with ->write_iter() we're
>> spending 8% of the time in _copy_from_iter(), and 4% in
>> new_sync_write(). That's obviously not there at all for the first case.
>> Both have about 4% in eventfd_write(). Non-iter case spends 1% in
>> copy_from_user().
>>
>> Finally with your branch pulled in as well, iow using ->write_iter() for
>> eventfd and your iov changes:
>>
>> Executed in  485.26 millis    fish           external
>>    usr time  103.09 millis   70.00 micros  103.03 millis
>>    sys time  382.18 millis   83.00 micros  382.09 millis
>>
>> Executed in  485.16 millis    fish           external
>>    usr time  104.07 millis   69.00 micros  104.00 millis
>>    sys time  381.09 millis   94.00 micros  381.00 millis
>>
>> and there's no real difference there. We're spending less time in
>> _copy_from_iter() (8% -> 6%) and less time in new_sync_write(), but
>> doesn't seem to manifest itself in reduced runtime.
> 
> Interesting... do you have instruction-level profiles for _copy_from_iter()
> and new_sync_write() on the last of those trees?

Attached output of perf annotate <func> for that last run.

-- 
Jens Axboe