From: "Morten Brørup" <mb@smartsharesystems.com>
To: "Thomas Monjalon" <thomas@monjalon.net>,
"David Marchand" <david.marchand@redhat.com>
Cc: <dev@dpdk.org>, <stephen@networkplumber.org>,
<bruce.richardson@intel.com>
Subject: RE: [PATCH v3 0/2] allow creating thread with real-time priority
Date: Thu, 26 Oct 2023 16:50:01 +0200 [thread overview]
Message-ID: <98CBD80474FA8B44BF855DF32C47DC35E9EF9C@smartserver.smartshare.dk> (raw)
In-Reply-To: <5410427.Sb9uPGUboI@thomas>
> From: Thomas Monjalon [mailto:thomas@monjalon.net]
> Sent: Thursday, 26 October 2023 16.31
>
> 26/10/2023 16:08, Morten Brørup:
> > > From: Thomas Monjalon [mailto:thomas@monjalon.net]
> > > Sent: Thursday, 26 October 2023 16.05
> > >
> > > 26/10/2023 15:57, Morten Brørup:
> > > > > From: Morten Brørup [mailto:mb@smartsharesystems.com]
> > > > > Sent: Thursday, 26 October 2023 15.45
> > > > >
> > > > > > From: Thomas Monjalon [mailto:thomas@monjalon.net]
> > > > > > Sent: Thursday, 26 October 2023 15.37
> > > > > >
> > > > > > 25/10/2023 18:31, Thomas Monjalon:
> > > > > > > Real-time thread priority was been forbidden on Unix
> > > > > > > because of problems they can cause.
> > > > > > > Warnings and helpers are added to avoid deadlocks,
> > > > > > > so real-time can be allowed on all systems.
> > > > > >
> > > > > > Unit test is failing:
> > > > > > DPDK:fast-tests / threads_autotest TIMEOUT 600.01 s
> > > > > >
> > > > > > It is seen in only 1 target (maybe the failure occurence is random):
> > > > > > Debian 11 (Buster) (ARM) | PASS
> > > > > > Fedora 37 (ARM) | PASS
> > > > > > CentOS Stream 9 (ARM) | FAIL
> > > > > > Fedora 38 (ARM) | PASS
> > > > > > Fedora 38 (ARM Clang) | PASS
> > > > > > Ubuntu 20.04 (ARM) | PASS
> > > > > >
> > > > > > I need to send a v4 with new implementation and better comments.
> > > > > > The Unix sleep will be upgraded from 1 ns to 1 us in case it makes a
> > > > > > difference.
> > > > >
> > > > > It will not make a difference. The kernel will go through the sleeping
> > > steps,
> > > > > then wake up again and see the real-time thread is ready to run, and
> then
> > > > > immediately schedule it.
> > > > >
> > > > > For testing purposes, consider sleeping 10 milliseconds or something
> > > > > significant like that.
> > > >
> > > > A bit more details...
> > > >
> > > > In our recent tests, nanosleep() itself took around 50 us. So you need
> to
> > > sleep longer than that for your thread not to be runnable when the
> nanosleep()
> > > wakes up again, because 50 us has already passed in "nanosleep overhead".
> > > > 10 milliseconds provides plenty of margin, and corresponds to 10 jiffies
> on
> > > a 1000 Hz kernel. (I don't know if it makes any difference for the kernel
> > > scheduler if the timer crosses a jiffy border or not.)
> > >
> > > 10 ms looks like an eternity.
> >
> > Agree. It is only for functional testing, not for production!
>
> Realtime thread won't make any sense if we have to insert a long sleep.
It seems David came to our rescue here!
I have just tried running our test again with prctl(PR_SET_TIMERSLACK) of 1 ns, and the nanosleep(1 ns) delay dropped from ca. 50 us to ca. 2.5 us.
The timeout parameter to epoll_wait() is in milliseconds, which is useless for low-latency.
Perhaps real-time threads can be used with epoll() combined with timerfd for nanosecond resolution timeout.
> We need to find a good sleep value or give up with real-time threads.
> (note I'm not sure how much it is useful)
Only the application developer knows how much delay is acceptable. Which is why I mentioned that the new yield functions should document the delay.
>
> > > I will try.
> > > (Anyway I did a mistake when sending v4)
>
> I've sent a trial with 1 ms.
>
next prev parent reply other threads:[~2023-10-26 14:50 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-24 12:54 [PATCH] eal/unix: " Thomas Monjalon
2023-10-24 13:55 ` Morten Brørup
2023-10-24 16:04 ` Stephen Hemminger
2023-10-25 13:15 ` Thomas Monjalon
2023-10-25 13:34 ` Bruce Richardson
2023-10-25 13:44 ` Thomas Monjalon
2023-10-25 15:08 ` Stephen Hemminger
2023-10-25 15:14 ` Bruce Richardson
2023-10-25 15:18 ` Thomas Monjalon
2023-10-25 15:32 ` Thomas Monjalon
2023-10-25 15:13 ` [PATCH v2] " Thomas Monjalon
2023-10-25 15:37 ` Stephen Hemminger
2023-10-25 16:46 ` Thomas Monjalon
2023-10-25 17:54 ` Morten Brørup
2023-10-25 21:33 ` Stephen Hemminger
2023-10-26 7:33 ` Morten Brørup
2023-10-26 16:32 ` Stephen Hemminger
2023-10-26 17:07 ` Morten Brørup
2023-10-26 0:00 ` Stephen Hemminger
2023-10-25 16:31 ` [PATCH v3 0/2] " Thomas Monjalon
2023-10-25 16:31 ` [PATCH v3 1/2] eal: add thread yield functions Thomas Monjalon
2023-10-25 16:40 ` Bruce Richardson
2023-10-25 17:06 ` Thomas Monjalon
2023-10-25 18:07 ` Morten Brørup
2023-10-25 16:31 ` [PATCH v3 2/2] eal/unix: allow creating thread with real-time priority Thomas Monjalon
2023-10-26 13:36 ` [PATCH v3 0/2] " Thomas Monjalon
2023-10-26 13:44 ` Morten Brørup
2023-10-26 13:57 ` Morten Brørup
2023-10-26 14:04 ` Thomas Monjalon
2023-10-26 14:08 ` Morten Brørup
2023-10-26 14:30 ` Thomas Monjalon
2023-10-26 14:50 ` Morten Brørup [this message]
2023-10-26 14:59 ` Morten Brørup
2023-10-26 15:54 ` Bruce Richardson
2023-10-26 16:07 ` Thomas Monjalon
2023-10-26 16:50 ` Morten Brørup
2023-10-26 19:51 ` Thomas Monjalon
2023-10-27 7:19 ` Morten Brørup
2023-10-26 16:28 ` Stephen Hemminger
2023-10-26 19:55 ` Thomas Monjalon
2023-10-26 21:10 ` Stephen Hemminger
2023-10-26 14:10 ` David Marchand
2023-10-26 13:37 ` [PATCH v4 " Thomas Monjalon
2023-10-26 13:37 ` [PATCH v4 1/2] eal: add thread yield functions Thomas Monjalon
2023-10-26 13:37 ` [PATCH v4 2/2] eal/unix: allow creating thread with real-time priority Thomas Monjalon
2023-10-26 14:19 ` [PATCH v5 0/2] " Thomas Monjalon
2023-10-26 14:19 ` [PATCH v5 1/2] eal: add thread yield functions Thomas Monjalon
2023-10-26 14:19 ` [PATCH v5 2/2] eal/unix: allow creating thread with real-time priority Thomas Monjalon
2023-10-26 20:00 ` [PATCH v5 0/2] " Thomas Monjalon
2023-10-26 23:35 ` Tyler Retzlaff
2023-10-27 8:08 ` [PATCH v6 1/1] eal/unix: " Thomas Monjalon
2023-10-27 8:45 ` Morten Brørup
2023-10-27 9:11 ` Thomas Monjalon
2023-10-27 18:15 ` Stephen Hemminger
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