tcp_isn_tick() / dummynet() callout madness ?
Mike Silbersack
silby at silby.com
Sun Jan 30 13:23:37 PST 2005
On Sun, 30 Jan 2005, Colin Percival wrote:
> Robert Watson wrote:
>> since the callout_reset() is one of the more
>> expensive parts of this code, Colin has been looking at some locking
>> optimizations to lower the cost.
>
> To elaborate somewhat: I think I can avoid the spinlock cost when
> callouts reset themselves (which is the case here). However, while
> this will reduce the time spent in the callouts themselves, it's
> really only a 50% solution -- softclock locks and unlocks the callout
> spin lock each time it launches a callout. If we're spending 5% of
> our cpu time in these two callouts, then they're actually responsible
> for using 10% of our cpu time; I think I can cut that in half, but in
> the end we can't avoid the cost of a mtx_lock_spin / mtx_unlock_spin
> pair (in softclock) for each callout.
>
> Colin Percival
Is there any way to get around that cost with some relatively simple
change to the callout API? Just a few places in the kernel account for
most of the use of callouts, so even if a rewrite of those would be
necessary, it should pay off.
Or, potentially crazy idea here; instead of incurring the cost of a
spinlock to remove a callout entry from a bucket, could you do some atomic
operation to mark that entry as done, and then only remove those entries
once and a while?
I guess if spinlocks weren't so expensive, this wouldn't be a big deal...
why do they need to be spinlocks? :)
Mike "Silby" Silbersack
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