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Re: [Qemu-devel] Using aio_poll for timer carrier threads

From: Alex Bligh
Subject: Re: [Qemu-devel] Using aio_poll for timer carrier threads
Date: Mon, 19 Aug 2013 14:58:24 +0100

On 19 Aug 2013, at 14:21, Paolo Bonzini wrote:

> I like Stefan's patches to make the timer list thread-safe, especially
> if we can optimize it (with RCU?) to make the read side lockless.

We will have to be careful on the write side. I'm trying to
work out just how slow this would with Paolo's RCU patches.
If call_rcu is used (or synchronize_rcu after the entire list
has been traversed, all timers updated etc), how bad is it?

There is a particular concern for  for repeating timers where two writes
are done (firstly to remove the timer from the list, then next
inside the timer routine to add it back to the list). As timers
are only called by a single thread, it might be worth having
some form of timer API which specifies a timer should be
called every X nano/micro/milli/seconds, or just once in X
nano/micro/milli/seconds time, which would save the inevitable
call to read the clock value first, and would speed things up
if write locking was slow.

Alex Bligh

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