Messages in this thread | | | Date | Thu, 06 Aug 2009 13:18:47 +0300 | From | Avi Kivity <> | Subject | Re: [RFC] respect the referenced bit of KVM guest pages? |
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On 08/06/2009 01:08 PM, Andrea Arcangeli wrote: > After some conversation it seems reactivating on large systems > generates troubles to the VM as young bit have excessive time to be > reactivated, giving troubles to shrink active list. I see that, so > then the check should be still nuked, but the unconditional > deactivation should happen instead. Otherwise it's trivial to put the > VM to its knees and DoS it with a simple mmap of a file with MAP_EXEC > as parameter of mmap. My whole point is that deciding if activating or > deactivating pages can't be in function of VM_EXEC, and clearly it > helps on desktops but then it probably is a signal that the VM isn't > good enough by itself to identify the important working set using > young bits and stuff on desktop systems, and if there's a good reason > to not activate, we shouldn't activate the VM_EXEC either as anything > and anybody can generate a file mapping with VM_EXEC set... >
Reasonable; if you depend on a hint from userspace, that hint can be used against you.
> Likely we need a cut-off point, if we detect it takes more than X > seconds to scan the whole active list, we start ignoring young bits, > as young bits don't provide any meaningful information then and they > just hang the VM in preventing it to shrink active list and looping > over it endlessy with million pages inside that list. But on small > systems if inactive list is short it may be too quick to just clear > the young bit and only giving it time to be re-enabled in inactive > list. That may be the source of the problem. Actually I'm speculating > here, because I barely understood that this is swapin... not sure > exactly what this regression is about but testing the patch posted is > good idea and it will tell us if we just need to dynamically > differentiating the algorithm between large and small systems and start > ignoring young bits only at some point. >
How about, for every N pages that you scan, evict at least 1 page, regardless of young bit status? That limits overscanning to a N:1 ratio. With N=250 we'll spend at most 25 usec in order to locate one page to evict.
-- error compiling committee.c: too many arguments to function
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