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Date: Thu, 1 Dec 2016 09:35:27 -0500 (EST)
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] Linux kernel polling for QEMU
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To: Avi Kivity <avi@scylladb.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>, Willem de Bruijn <willemb@google.com>, Fam Zheng <famz@redhat.com>, Eliezer Tamir <eliezer.tamir@linux.intel.com>, Eric Dumazet <eric.dumazet@gmail.com>, "Michael S. Tsirkin" <mst@redhat.com>, qemu-devel@nongnu.org, Jens Axboe <axboe@fb.com>, Christian Borntraeger <borntraeger@de.ibm.com>, Davide Libenzi <davidel@xmailserver.org>, Christoph Hellwig <hch@lst.de>


> > > Maybe we could do the same for sockets?  When data is available on a
> > > socket (or when it becomes writable), write to a user memory location.
> > >
> > > I, too, have an interest in polling; in my situation most of the polling
> > > happens in userspace.
> >
> > You are trying to improve on the latency of non-blocking
> > ppoll(2)/epoll_wait(2) call?
> 
> Yes, but the concern is for throughput, not latency.
> 
> My main loop looks like
> 
>     execute some tasks
>     poll from many sources
> 
> Since epoll_wait(..., 0) has non-trivial costs, I have to limit the
> polling rate, and even so it shows up in the profile.  If the cost were
> lower, I could poll at a higher frequency, resulting in lower worst-case
> latencies for high-priority tasks.

IMHO, the ideal model wouldn't enter the kernel at all unless you _want_
to go to sleep.

Paolo