Re: [RFC][PATCH 0/8] Critical Page Pool

[Matthew Dobson <colpatch@us.ibm.com>]

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.

-Matt

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