Re: [tor-relays] FreeBSD 13.1: clock_gettime(CLOCK_MONOTONIC_FAST) ~ 50 % performance gain

["Alex Xu \(Hello71\) via tor-relays" <tor-relays@xxxxxxxxxxxxxxxxxxxx>]

Excerpts from Andreas Kempe's message of June 21, 2022 11:50 am:
> Hello everyone,
> 
> I was doing some profiling on my two relays running on FreeBSD 13.1
> and noticed that they were spending a lot of time in clock_gettime()
> which prompted me to have a look at the implementation.
> 
> Time implementation
> ===================
> 
> The time implementation is abstracted in src/lib/time/compat_time.c
> where different mechanisms are used for different operating systems.
> On Linux CLOCK_MONOTONIC_COARSE is a clock that gives worse precision
> than CLOCK_MONOTONIC, but is faster and the abstraction layer checks
> for its presense and provides more performat less precise time where
> applicable.
> 
> On FreeBSD, there is also a fast monotonic time source available
> called CLOCK_MONOTONIC_FAST. In the header file
> src/lib/time/compat_time.h, a comment references this clock, but it is
> not used. I thought it might be worth a shot seeing what difference it
> would make if I enable the use of CLOCK_MONOTONIC_FAST on FreeBSD and
> on the VM where I run my two FreeBSD relays, the difference was
> stunning.
> 
> I made did a quick patch simply replacing CLOCK_MONOTONIC_COARSE with
> CLOCK_MONOTONIC_FAST, see patches attached, compiled and tested.
> Tracing system calls to make sure the correct call was being used,
> which it was.

According to https://www.freebsd.org/cgi/man.cgi?query=clock_gettime, 
FreeBSD 13.1 has CLOCK_MONOTONIC_COARSE, which it says is an alias of 
CLOCK_MONOTONIC_FAST for compatibility with other systems. I suppose Tor 
could add #if !defined(CLOCK_MONOTONIC_COARSE) && 
defined(CLOCK_MONOTONIC_FAST) #define CLOCK_MONOTONIC_COARSE 
CLOCK_MONOTONIC_FAST, but I'm not sure how useful that would be. OpenBSD 
and NetBSD don't seem to define either. Perhaps something like that 
would be appropriate for a FreeBSD ports patch.

Cheers,
Alex.
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