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Re: [Qemu-devel] [RFC 0/8] arm AioContext with its own timer stuff


From: Alex Bligh
Subject: Re: [Qemu-devel] [RFC 0/8] arm AioContext with its own timer stuff
Date: Wed, 24 Jul 2013 09:01:22 +0100

Paolo,

--On 24 July 2013 09:43:28 +0200 Paolo Bonzini <address@hidden> wrote:

Most 'reasonable' POSIX compliant operating systems have ppoll

Really?  I could find no manpages for any of Solaris and *BSD.

OK I shall (re)research that then! I suppose select() / pselect() is
an alternative when there are few FDs.

, my concern was mainly about
Windows (which I know very little about), as there does not appear
to be a nanosecond or even microsecond alternative to
WaitForMultipleObjects. However, articles like this:

http://social.msdn.microsoft.com/Forums/vstudio/en-US/e8a7cb1e-9edd-4ee3
-982e-f66b7bf6ae44/improve-accuracy-waitforsingleobject

suggest that WaitFor{Single,Multiple}Objects can have pretty
appalling latency anyway (100ms!), and there's no evidence that's
limited by making one of the FDs (or objects) ready.

... especially when making one of the FDs ready would likely have the
same latency in some internal Windows thread that implements timers.

In these
circumstances, I'd question whether we gain anything by worrying
about timer resolution.

Part of it should be fixed by os_setup_early_signal_handling.

This is corroborated by the fact that without
os_setup_early_signal_handling Wine always works, and Windows breaks.

This:
 http://www.windowstimestamp.com/description
suggests that whilst WaitForMultipleEvents has a millisecond timeout, one can (see section 3.2) use these to wait for an object which is itself a timer and expires with - in this case - 100ns resolution which is probably enough.

Again I know nothing about Windows so this may be completely wrong.

--
Alex Bligh



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