Messages in this thread | | | Date | Wed, 14 Dec 2005 07:55:24 -0800 | From | Matthew Dobson <> | Subject | Re: [RFC][PATCH 0/6] Critical Page Pool |
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Pavel Machek wrote: > Hi! > > >>The overall purpose of this patch series is to all a system administrator >>to reserve a number of pages in a 'critical pool' that is set aside for >>situations when the system is 'in emergency'. It is up to the individual >>administrator to determine when his/her system is 'in emergency'. This is >>not meant to (necessarily) anticipate OOM situations, though that is >>certainly one possible use. The purpose this was originally designed for >>is to allow the networking code to keep functioning despite the sytem >>losing its (potentially networked) swap device, and thus temporarily >>putting the system under exreme memory pressure. > > > I don't see how this can ever work. > > How can _userspace_ know about what allocations are critical to the > kernel?!
Well, it isn't userspace that is determining *which* allocations are critical to the kernel. That is statically determined at compile time by using the flag __GFP_CRITICAL on specific *kernel* allocations. Sridhar, cc'd on this mail, has a set of patches that sprinkle the __GFP_CRITICAL flag throughout the networking code to take advantage of this pool. Userspace is in charge of determining *when* we're in an emergency situation, and should thus use the critical pool, but not *which* allocations are critical to surviving this emergency situation.
> And as you noticed, it does not work for your original usage case, > because reserved memory pool would have to be "sum of all network > interface bandwidths * ammount of time expected to survive without > network" which is way too much.
Well, I never suggested it didn't work for my original usage case. The discussion we had is that it would be incredibly difficult to 100% iron-clad guarantee that the pool would NEVER run out of pages. But we can size the pool, especially given a decent workload approximation, so as to make failure far less likely.
> If you want few emergency pages for some strange hack you are doing > (swapping over network?), just put swap into ramdisk and swapon() it > when you are in emergency, or use memory hotplug and plug few more > gigabytes into your machine. But don't go introducing infrastructure > that _can't_ be used right.
Well, that's basically the point of posting these patches as an RFC. I'm not quite so delusional as to think they're going to get picked up right now. I was, however, hoping for feedback to figure out how to design infrastructure that *can* be used right, as well as trying to find other potential users of such a feature.
Thanks!
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