Re: [PATCH v1 0/3] reduce tty latency

From: Greg Kroah-Hartman
Date: Thu Jan 10 2019 - 08:51:58 EST


On Thu, Jan 10, 2019 at 11:12:29AM +0100, Oleksij Rempel wrote:
> This patch set is reducing latency on tty path.
> For testing I used hackbench running on all cores of 4 core system and
> high prioritized application sending and receiving packets over tty interface
> with loop-back adapter.

Odd, all of these ended up in my spam folder, you might want to check
your server email settings...

> Results without this patches:
> latency histogram:
> 0 ... < 250 usec : 1933104 transmissions
> 250 ... < 500 usec : 21339 transmissions
> 500 ... < 750 usec : 8952 transmissions
> 750 ... < 1000 usec : 6226 transmissions
> 1000 ... < 1500 usec : 7688 transmissions
> 1500 ... < 2000 usec : 5236 transmissions
> 2000 ... < 5000 usec : 11724 transmissions
> 5000 ... < 10000 usec : 3588 transmissions
> 10000 ... < 50000 usec : 2123 transmissions
> 50000 ... < 1000000 usec : 20 transmissions
> >= 1000000 usec : 0 transmissions
>
> Test results after this patches:
> min latency: 0 sec : 75 usec
> max latency: 0 sec : 125 usec
> average latency: 81 usec
> latency measure cycles overall: 79000000
> latency histogram:
> 0 ... < 250 usec : 79000000 transmissions
> 250 ... < 500 usec : 0 transmissions
> 500 ... < 750 usec : 0 transmissions
> 750 ... < 1000 usec : 0 transmissions
> 1000 ... < 1500 usec : 0 transmissions
> 1500 ... < 2000 usec : 0 transmissions
> 2000 ... < 5000 usec : 0 transmissions
> 5000 ... < 10000 usec : 0 transmissions
> 10000 ... < 50000 usec : 0 transmissions
> 50000 ... < 1000000 usec : 0 transmissions
> >= 1000000 usec : 0 transmissions
> average no. of read calls to assemble the packet: 1

Like Linus said, who runs a real-world system that cares about this
latency measurement?

Yes, it might be fun for odd benchmarks to show the value of one RT
patchset/OS vs. another one, but this change can cause real issues in
real systems that do real, non-serial-loopback work.

thanks,

greg k-h