[etherlab-dev] Sanity check: default send interval

[Frank Heckenbach]

Gavin Lambert wrote:

> Just noticed something that I think is incorrect, but I thought I'd ask for
> a sanity check from the group in case I'm missing something.
> 
> In master/master.c's ec_master_init (line 213 in my patched version; line
> 211 in stable-1.5) there's the following line:
> 
>     ec_master_set_send_interval(master, 1000000 / HZ);
> 
> According to the definition further down and the docs, that second parameter
> is supposed to be the time between master application cycles in
> microseconds, which is used to calculate a few queue sizes and also to
> control the master thread sleep time if EC_USE_HRTIMER is defined (via
> configure --enable-hrtimer).
> 
> Doesn't the use of HZ above mean that this is actually calculating "how many
> seconds is 1000000 jiffies"

Which is the same as "how many microseconds is 1 jiffy". So sleeping
for this many microseconds is as close as possible to the
non-hrtimer code which sleeps for 1 jiffy.

> On a somewhat related note, is there some guidance for when using
> --enable-hrtimer is good or bad?  Does it relate just to whether the hrtimer
> is present/trusted or is there something more subtle?  In terms of the code,
> it just seems to be the difference between a "sleep for X time" vs. a "sleep
> for 1 quantum", so it *seems* like --enable-hrtimer is good for reducing CPU
> usage.

The hrtimer code is a little clearer (IMHO) as the ugliness is
concentrated in one function (ec_master_nanosleep) (which BTW also
helped me with my userspace port as I only had to replace this one
function rather than the several set_current_state/schedule
constructs in the non-hrtimer code).

More importantly, you need hrtimers if you want (master) cycle times
shorter than 1 jiffy. If you don't need them, I don't think there's
a big practical advantage in using hrtimers.

Regards,
Frank

-- 
Dipl.-Math. Frank Heckenbach <f.heckenbach at fh-soft.de>
Stubenlohstr. 6, 91052 Erlangen, Germany, +49-9131-21359
Systems Programming, Software Development, IT Consulting