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From: Peter Zijlstra on 6 Apr 2010 11:00
On Tue, 2010-04-06 at 07:47 -0700, Ulrich Drepper wrote:
> On Tue, Apr 6, 2010 at 01:48, Peter Zijlstra <peterz(a)infradead.org>
> wrote:
> > try
> > spin
> > try
> > syscall
>
> This is available for a long time in the mutex implementation
> (PTHREAD_MUTEX_ADAPTIVE_NP mutex type). It hasn't show much
> improvement if any. There were some people demanding this support for
> as far as I know they are not using it now. This is adaptive
> spinning, learning from previous calls how long to wait. But it's
> still unguided. There is no way to get information like "the owner
> has been descheduled".

That's where the FUTEX_LOCK thing comes in, it does all those, the above
was a single spin loop to amortize the syscall overhead.

I wouldn't make it any more complex than a single pause ins, syscalls
are terribly cheap these days.

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From: Alan Cox on 10 Apr 2010 19:40
> The non-swappable part will be tricky. One doesn't know how many
> threads will be created in a process. This mechanism shouldn't put an
> arbitrary limit in place. So where to allocate the memory? Perhaps
> it's better to implicitly mark the memory page pointed to by the new
> syscall as non-swappable? This could mean one page per thread...

You only need one page per 4096 threads or so if you make it create the
page on the first request, tied to say the signals and the like in
threaded tasks, and you then allocate 'slots' in the page for future
calls until you've got 4096 live.

ie you'd see something like

addr = set_tid_notifier_addr();

[1st call
map page at x to x + 4095, probably R/O]
[returns x]

next thread
addr = set_tid_notifier_addr()

[returns x + 1]

Alan
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