Re: [RFC 0/7] Postphone reclaim laundry to write at high water marks

[Christoph Lameter <clameter@sgi.com>]

On Wed, 22 Aug 2007, Peter Zijlstra wrote:

> > That is an extreme case that AFAIK we currently ignore and could be 
> > avoided with some effort.
> 
> Its not extreme, not even rare, and its handled now. Its what
> PF_MEMALLOC is for.

No its not. If you have all pages allocated as anonymous pages and your 
writeout requires more pages than available in the reserves then you are 
screwed either way regardless if you have PF_MEMALLOC set or not.

> > The initial PF_MEMALLOC patchset seems to be 
> > still enough to deal with your issues.
> 
> Take the anonyous workload, user-space will block once the page
> allocator hits ALLOC_MIN. Network will be able to receive until
> ALLOC_MIN|ALLOC_HIGH - if the completion doesn't arrive by then it will
> start dropping all packets until there is memory again. But userspace is
> wedged and hence will not consume the network traffic, hence we
> deadlock.
> 
> Even if there is something to reclaim initially, if the pressure
> persists that can eventually be exhausted.

Sure ultimately you will end up with pages that are all unreclaimable if 
you reclaim all reclaimable memory.

> > multiple critical tasks on various devices that have various memory needs. 
> > So multiple critical spots can happen concurrently in multiple 
> > application contexts.
> 
> yes, reclaim can be unbounded concurrent, and that is one of the
> (theoretically) major problems we currently have.

So your patchset is not fixing it?

> > We have that with PF_MEMALLOC.
> 
> Exactly. But if you recognise the need for PF_MEMALLOC then what is this
> argument about?

The PF_MEMALLOC patchset f.e. is about avoiding to go out of 
memory when there is still memory available even if we are doing a 
PF_MEMALLOC allocation and would OOM otherwise.

> Networking can currently be seen as having two states:
> 
>  1 receive packets and consume memory
>  2 drop all packets (when out of memory)
> 
> I need a 3rd state:
> 
>  3 receiving packets but not consuming memory

So far a good idea. If you are not consuming memory then why are the 
allocators involved?
 
> Now, I need this state when we're in PF_MEMALLOC territory, because I
> need to be able to process an unspecified amount of network traffic in
> order to receive the writeout completion.
> 
> In order to operate this 3rd network state, some memory is needed in
> which packets can be received and when deemed not important freed and
> reused.
> 
> It needs a bounded amount of memory in order to process an unbounded
> amount of network traffic.
> 
> What exactly is not clear about this? If you accept the need for
> PF_MEMALLOC you surely must also agree that at the point you're using it
> running reclaim is useless.

Yes looks like you would like to add something to the network layer to 
filter important packets. As long as you stay within PF_MEMALLOC 
boundaries you can allocate and throw packets away. If you want to have a 
reserve that is secure and just for you then you need to take it away from 
the reserves (which in turn will lead reclaim to restore them).

> > > Also, failing a memory allocation isn't bad, why are you so worried
> > > about that? It happens all the time.
> > 
> > Its a performance impact and plainly does not make sense if there is 
> > reclaimable memory availble. The common action of the vm is to reclaim if 
> > there is a demand for memory. Now we suddenly abandon that approach?
> 
> I'm utterly confused by this, on one hand you recognise the need for
> PF_MEMALLOC but on the other hand you're saying its not needed and
> anybody needing memory (even reclaim itself) should use reclaim.

The VM reclaims memory on demand but in exceptional limited cases where we 
cannot do so we use the reserves. I am sure you know this.


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