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Date: Fri, 28 Mar 2008 23:58:02 -1000 (HST)
From: Jeff Roberson <jroberson@chesapeake.net>
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To: Hans Petter Selasky <hselasky@c2i.net>
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Cc: arch@freebsd.org, freebsd-arch@freebsd.org
Subject: Re: timeout/callout small step forward
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On Sat, 29 Mar 2008, Hans Petter Selasky wrote:

> Hi,
>
> How does this patch handle when multiple callouts use the same mutex ? Imagine
> that two threads are about to lock the same mutex, then the second thread
> will get nowhere while waiting for the first thread to execute the callback.

You are worried that there will be too much contention or that there is 
some correctness issue?

If you're worried about contention, it would seem that most callers use 
some per-object mutex for the callout.  So there isn't likely to be 
much contention among callouts.

>
> I think the solution is that callbacks are put in a tree. Giant locked
> callbacks go all into the same tree. Else the user of callouts is responsible
> for making up the tree.
>
> struct callout {
> 	struct thread *exec_td;
> 	struct callout *parent;
> };
>
> struct callout *c;
>
> while (c->parent != NULL) {
> 	c = c->parent;
> }
>
> if (c->exec_td != NULL) {
> 	callout should go into the same thread;
> } else {
> 	pick the next free thread and set "c->exec_td"
> }
>
> Callouts that belong to the same tree are run from the same thread. This does
> not only apply for callouts, but also multithreaded node systems ...
>
> Yours
>  --HPS
>