Re: [PATCH] block: reexpand iov_iter after read/write

[Al Viro <viro@zeniv.linux.org.uk>]

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?