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Message-ID: <20150108184201.GB13974@cloud>
Date: Thu, 8 Jan 2015 10:42:01 -0800
From: josh@...htriplett.org
To: Andy Lutomirski <luto@...capital.net>
Cc: Miklos Szeredi <mszeredi@...e.cz>, Fam Zheng <famz@...hat.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
Alexander Viro <viro@...iv.linux.org.uk>,
Andrew Morton <akpm@...ux-foundation.org>,
Juri Lelli <juri.lelli@...il.com>, Zach Brown <zab@...bo.net>,
David Drysdale <drysdale@...gle.com>,
Kees Cook <keescook@...omium.org>,
Alexei Starovoitov <ast@...mgrid.com>,
David Herrmann <dh.herrmann@...il.com>,
Dario Faggioli <raistlin@...ux.it>,
Theodore Ts'o <tytso@....edu>,
Peter Zijlstra <peterz@...radead.org>,
Vivek Goyal <vgoyal@...hat.com>,
Mike Frysinger <vapier@...too.org>,
Heiko Carstens <heiko.carstens@...ibm.com>,
Rasmus Villemoes <linux@...musvillemoes.dk>,
Oleg Nesterov <oleg@...hat.com>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Fabian Frederick <fabf@...net.be>,
"David S. Miller" <davem@...emloft.net>,
Linux FS Devel <linux-fsdevel@...r.kernel.org>,
Linux API <linux-api@...r.kernel.org>,
Paolo Bonzini <pbonzini@...hat.com>,
Stefan Hajnoczi <stefanha@...hat.com>,
"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Subject: Re: [PATCH 0/3] epoll: Add epoll_pwait1 syscall
On Thu, Jan 08, 2015 at 09:57:24AM -0800, Andy Lutomirski wrote:
> On Thu, Jan 8, 2015 at 1:12 AM, Miklos Szeredi <mszeredi@...e.cz> wrote:
> > On Thu, 2015-01-08 at 16:25 +0800, Fam Zheng wrote:
> >> Applications could use epoll interface when then need to poll a big number of
> >> files in their main loops, to achieve better performance than ppoll(2). Except
> >> for one concern: epoll only takes timeout parameters in microseconds, rather
> >> than nanoseconds.
> >>
> >> That is a drawback we should address. For a real case in QEMU, we run into a
> >> scalability issue with ppoll(2) when many devices are attached to guest, in
> >> which case many host fds, such as virtual disk images and sockets, need to be
> >> polled by the main loop. As a result we are looking at switching to epoll, but
> >> the coarse timeout precision is a trouble, as explained below.
> >>
> >> We're already using prctl(PR_SET_TIMERSLACK, 1) which is necessary to implement
> >> timers in the main loop; and we call ppoll(2) with the next firing timer as
> >> timeout, so when ppoll(2) returns, we know that we have more work to do (either
> >> handling IO events, or fire a timer callback). This is natual and efficient,
> >> except that ppoll(2) itself is slow.
> >>
> >> Now that we want to switch to epoll, to speed up the polling. However the timer
> >> slack setting will be effectively undone, because that way we will have to
> >> round up the timeout to microseconds honoring timer contract. But consequently,
> >> this hurts the general responsiveness.
> >>
> >> Note: there are two alternatives, without changing kernel:
> >>
> >> 1) Leading ppoll(2), with the epollfd only and a nanosecond timeout. It won't
> >> be slow as one fd is polled. No more scalability issue. And if there are
> >> events, we know from ppoll(2)'s return, then we do the epoll_wait(2) with
> >> timeout=0; otherwise, there can't be events for the epoll, skip the following
> >> epoll_wait and just continue with other work.
> >>
> >> 2) Setup and add a timerfd to epoll, then we do epoll_wait(..., timeout=-1).
> >> The timerfd will hopefully force epoll_wait to return when it timeouts, even if
> >> no other events have arrived. This will inheritly give us timerfd's precision.
> >> Note that for each poll, the desired timeout is different because the next
> >> timer is different, so that, before each epoll_wait(2), there will be a
> >> timerfd_settime syscall to set it to a proper value.
> >>
> >> Unfortunately, both approaches require one more syscall per iteration, compared
> >> to the original single ppoll(2), cost of which is unneglectable when we talk
> >> about nanosecond granularity.
>
> I'd like to see a more ambitious change, since the timer isn't the
> only problem like this. Specifically, I'd like a syscall that does a
> list of epoll-related things and then waits. The list of things could
> include, at least:
>
> - EPOLL_CTL_MOD actions: level-triggered epoll users are likely to
> want to turn on and off their requests for events on a somewhat
> regular basis.
>
> - timerfd_settime actions: this allows a single syscall to wait and
> adjust *both* monotonic and real-time wakeups.
>
> Would this make sense? It could look like:
>
> int epoll_mod_and_pwait(int epfd,
> struct epoll_event *events, int maxevents,
> struct epoll_command *commands, int ncommands,
> const sigset_t *sigmask);
That's a complicated syscall. (And it also doesn't have room for the
flags argument.)
At that point, why not just have a syscall like this:
struct syscall {
unsigned long num;
unsigned long params[6];
};
int sys_many(size_t count, struct syscall *syscalls, int *results, unsigned long flags);
I think that has been discussed in the past.
Or, these days, that might be better done via eBPF, which would avoid
the need for flags like "return on error"; an eBPF program could decide
how to proceed after each call.
- Josh Triplett
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