Messages in this thread | | | Date | Fri, 7 Mar 2008 06:13:24 +0300 | From | Oleg Nesterov <> | Subject | Re: [RFC,PATCH 2/2] __group_complete_signal: fix? signal load-balancing |
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On 03/06, Roland McGrath wrote: > > > The comment says about load-balancing, but this is not what happens? > > Suppose that wants_signal(signal->curr_target) == T. In that case we > > always choose the same ->curr_target thread. Isn't it better to try > > to "spread" the signals over the thread group? > > The "load balancing" stuff was in the old multithreaded signals code (2.5) > from before I rearranged a lot of code to fix the main parts of the MT > semantics. Maybe it was Ingo who originally put that code in? I moved > everything around it to change the deterministic semantics, but I never > really gave any thought to the "performance feature". Perhaps it did > something different to begin with and bit-rot made it into the algorithm we > have that seems not so optimal . > > The current behavior hammers all the unblocked signals onto one thread > until it's scheduled out. For getting the signal delivered as quickly as > can be, it makes some sense to choose running threads (task_curr) over > threads blocked without signals already pending. So perhaps the same > thread that just ran a signal handler (maybe is still setting it up) really > is the preferable choice when it's on the CPU--at least in comparison to > another candidate thread that is not on a CPU. But it's not exactly doing > "load balancing". If several threads are running on CPUs, presumably it's > intended to spread several near-simultaneous signals across those CPUs. > > Perhaps Ingo has some thoughts on what the original plan is, or on what > desireable performance choices are now.
OK, thanks, please ignore this patch (it was more the question anyway).
So. currently the meaning of->curr_target is: remember the last thread we sent a signal, may help to avoid iterating over the thread group when the next signal is sent.
> If we're cleaning up, we can start by getting rid of the NULL check. > There's no reason to have it in this hot path. It should never come > up if we make copy_signal initialize sig->curr_target = tsk.
Can't understand why I didn't realize this while reading the code. Looks like a reasonable cleanup regardless.
Oleg.
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