Re: [pve-devel] [PATCH pve-cluster 1/1] notify: add common_template_data

[Thomas Lamprecht <t.lamprecht@proxmox.com>]

Am 28.03.25 um 09:28 schrieb Lukas Wagner:
> We of course can cache the FQDN, but realistically speaking, this is only called once per
> notification being sent, thus any real-world performance impact is absolutely tiny.

Not so sure about that in general, e.g. sending out notifications could
correlate with an overloaded system, and for really overloaded systems
things that are normally cheap suddenly ain't – e.g., on low memory
situations even a doing a plain fork+exec of a tiny binary can hang for
a long time then, socket operations like our helper does are definitively
less problematic (I think, as I did not evaluate it [0] and that's something
one can less easily experience directly compared to the former, where even
starting a new basic dash shell on such overloaded system can need minutes).

And as the notification system now also handles things like HA events it's
definitively part of the more critical systems which _can_ justify some extra
scrutiny. That said, switching to the get_fqdn method makes this indeed quite
cheap to get [0], so I'm fine with not doing any caching here for now, but
let's not underestimate the impact of such things too much, especially for
anything in critical chains that can be important in critical (load) situation
(as general strategy for all, as I'm really not thinking about you here, and
it certainly is a balance).

[0]: FWIW, I just did a quick evaluation of querying the fqdn 100 000 times
with the socket variant and the hostname one, this was done on a very healthy
system though, I'd expect that the fork+exec one degrades a lot worse with
higher cpu/memory pressure. Test and result:

# perl -wE 'use PVE::Tools; use Time::HiRes qw(gettimeofday tv_interval); my $t0 = [gettimeofday]; for(my $i = 0; $i <= 100_000; $i++) { my $fqdn = PVE::Tools::get_fqdn("nina"); } my $elapsed = tv_interval ( $t0, [gettimeofday]); say "elapsed (s): ". $elapsed;' 
elapsed (s): 0.436712

Same with 1 million runs gets me 4.368217 s, so seems to scale quite linearly.

# perl -wE 'use Time::HiRes qw(gettimeofday tv_interval); my $t0 = [gettimeofday]; for(my $i = 0; $i <= 100_000; $i++) { my $fqdn = `hostname -f`; } my $elapsed = tv_interval ( $t0, [gettimeofday]); say "elapsed: ". $elapsed;'      
elapsed (s): 82.484177

Same with 1 million runs gets me 577.653117 s, so not fully linearly, but
in any way about 188x and 132x times slower, respectively.


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