Re: [PATCH 0/3] epoll: Add epoll_pwait1 syscall

[josh@joshtriplett.org]

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@suse.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