Messages in this thread | | | Date | Sun, 20 Nov 2005 21:47:16 -0800 | From | Matthew Dobson <> | Subject | Re: [RFC][PATCH 0/8] Critical Page Pool |
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Chris Wright wrote: > * Matthew Dobson (colpatch@us.ibm.com) wrote: > >>/proc/sys/vm/critical_pages: write the number of pages you want to reserve >>for the critical pool into this file > > > How do you size this pool?
Trial and error. If you want networking to survive with no memory other than the critical pool for 2 minutes, for example, you pick a random value, block all other allocations (I have a test patch to do this), and send a boatload of packets at the box. If it OOMs, you need a bigger pool. Lather, rinse, repeat.
> Allocations are interrupt driven, so how to you > ensure you're allocating for the cluster network traffic you care about?
On the receive side, you can't. :( You *have* to allocate an skbuff for the packet, and only a couple levels up the networking 7-layer burrito can you tell if you can toss the packet as non-critical or keep it. On the send side, you can create a simple socket flag that tags all that socket's SEND requests as critical.
>>/proc/sys/vm/in_emergency: write a non-zero value to tell the kernel that >>the system is in an emergency state and authorize the kernel to dip into >>the critical pool to satisfy critical allocations. > > > Seems odd to me. Why make this another knob? How did you run to set this > flag if you're in emergency and kswapd is going nuts?
We did this because we didn't want __GFP_CRITICAL allocations dipping into the pool in the case of a transient low mem situation. In those cases we want to force the task to do writeback to get a page (as usual), so that the critical pool will be full when the system REALLY goes critical. We also open the in_emergency file when the app starts so that we can just write to it and don't need to try to open it when kswapd is going nuts.
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