On Fri 05-04-13 16:27:59, Wanpeng Li wrote: > On Fri, Apr 05, 2013 at 10:12:39AM +0200, Michal Hocko wrote: > >On Fri 05-04-13 07:41:23, Wanpeng Li wrote: > >> On Thu, Apr 04, 2013 at 06:17:46PM +0200, Michal Hocko wrote: > >> >On Thu 04-04-13 17:09:08, Wanpeng Li wrote: > >> >> order >= MAX_ORDER pages are only allocated at boot stage using the > >> >> bootmem allocator with the "hugepages=xxx" option. These pages are never > >> >> free after boot by default since it would be a one-way street(>= MAX_ORDER > >> >> pages cannot be allocated later), but if administrator confirm not to > >> >> use these gigantic pages any more, these pinned pages will waste memory > >> >> since other users can't grab free pages from gigantic hugetlb pool even > >> >> if OOM, it's not flexible. The patchset add hugetlb gigantic page pools > >> >> shrink supporting. Administrator can enable knob exported in sysctl to > >> >> permit to shrink gigantic hugetlb pool. > >> > > >> >I am not sure I see why the new knob is needed. > >> >/sys/kernel/mm/hugepages/hugepages-*/nr_hugepages is root interface so > >> >an additional step to allow writing to the file doesn't make much sense > >> >to me to be honest. > >> > > >> >Support for shrinking gigantic huge pages makes some sense to me but I > >> >would be interested in the real world example. GB pages are usually used > >> >in very specific environments where the amount is usually well known. > >> > >> Gigantic huge pages in hugetlb means h->order >= MAX_ORDER instead of GB > >> pages. ;-) > > > >Yes, I am aware of that but the question remains the same (and > >unanswered). What is the use case? > > The use case I can figure out is when memory pressure is serious and gigantic > huge pages pools still pin large number of free pages. Then this is a configuration issue. I understand that reboot is lame way to fix it but the gigantic pages usage is so specific that I would be really surprise if this kind of problem would pop out. I would also find surprising if those pages were unused. So the only use case I can figure out ATM is a hotplug scenario (after hugetlb migration patchset is ready) but even then I would find it more useful for in kernel usage (read hotplug). -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>