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Re: [Qemu-devel] [PATCH 1.1] coroutine: Avoid ucontext usage on i386 Lin

From: Peter Maydell
Subject: Re: [Qemu-devel] [PATCH 1.1] coroutine: Avoid ucontext usage on i386 Linux host
Date: Wed, 9 May 2012 21:46:28 +0100

On 9 May 2012 21:11, Jan Kiszka <address@hidden> wrote:
> OK. Then what about sigaltstack (once fixed)? Is it also slower? If not,
> can we converge over it?

sigaltstack is going to be significantly faster than the gthread
implementation (about the same speed as ucontext for coroutine
switch, a bit slower for coroutine creation).

It suffers from the same problem of being a code path which nobody
has seriously stress tested, though. And it does at least one thing
which is "not permitted by the standards but works anyway".

> I would really hate staying with this time bomb
> of broken RT signals unless someone tells me we will kick out all these
> coroutines rather sooner than later.

I think that regardless of the long term choice it would be
a pretty risky decision to switch coroutine implementation
this close to release when we have the option of just using
a different signal that avoids the bug.

Longer term (ie post 1.1) I'm strongly in favour of kicking
out coroutines, because I think there clearly is no single
solid portable implementation possible. C just isn't designed
to allow them; better not to try to swim against the current.

-- PMM

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