(Dropped the systemtap list since its mis-behaving, please leave it out on future postings) On Tue, 2011-04-19 at 11:56 +0530, Srikar Dronamraju wrote: > > > TODO: On massively threaded processes (or if a huge number of processes > > > share the same mm), there is a possiblilty of running out of slots. > > > One alternative could be to extend the slots as when slots are required. > > > > As long as you're single stepping things and not using boosted probes > > you can fully serialize the slot usage. Claim a slot on trap and release > > the slot on finish. Claiming can wait on a free slot since you already > > have the whole SLEEPY thing. > > > > Yes, thats certainly one approach but that approach makes every > breakpoint hit contend for spinlock. (Infact we will have to change it > to mutex lock (as you rightly pointed out) so that we allow threads to > wait when slots are not free). Assuming a 4K page, we would be taxing > applications that have less than 32 threads (which is probably the > default case). If we continue with the current approach, then we > could only add additional page(s) for apps which has more than 32 > threads and only when more than 32 __live__ threads have actually hit a > breakpoint. That very much depends on what you do, some folks think its entirely reasonable for processes to have thousands of threads. Now I completely agree with you that that is not 'normal', but then I think using Java isn't normal either ;-) Anyway, avoiding that spinlock/mutex for each trap isn't hard, avoiding a process wide cacheline bounce is slightly harder but still not impossible. With 32 slots in 4k you have 128 bytes to play with, all we need is a single bit per slot to mark it being in-use. If a task remembers what slot it used last and tries to claim that using an atomic test and set for that bit it will, in the 'normal' case, never contend on a process wide cacheline. In case it does find the slot taken, it'll have to go the slow route and scan for a free slot and possibly wait for one to become free. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href