Re: [PATCH] Document Linux Memory Policy

[glebn@voltaire.com (Gleb Natapov)]

On Thu, May 31, 2007 at 02:04:12PM +0300, Gleb Natapov wrote:
> On Thu, May 31, 2007 at 12:43:19PM +0200, Andi Kleen wrote:
> > 
> > > > The faulted page will use the memory policy of the task that faulted it 
> > > > in. If that process has numa_set_localalloc() set then the page will be 
> > > > located as closely as possible to the allocating thread.
> > > 
> > > Thanks. But I have to say this feels very unnatural.
> > 
> > What do you think is unnatural exactly? First one wins seems like a quite 
> > natural policy to me.
> No it is not (not always). I want to create shared memory for
> interprocess communication. Process A will write into the memory and
> process B will periodically poll it to see if there is a message there.
> In NUMA system I want the physical memory for this VMA to be allocated
> from node close to process B since it will use it much more frequently.
> But I don't want to pre-fault all pages in process B to achieve this
> because the region can be huge and because it doesn't guaranty much if
> swapping is involved. So numa_set_localalloc() looks like it achieves
> exactly this. Without this function I agree that the "first one wins" is
> very sensible assumption, but when each process stated it's preferences
> explicitly by calling the function it is not longer sensible to me as a
> user of the API. When you start to thing about how memory policy may be
OK now, rereading man page, I see that numa_tonode_memory() to achieve 
this without pre-faulting. A should now what CPU B is running on, but
this is a minor problem.

> implemented in the kernel and understand that memory policy is a
> property of an address space (is it?) and not a file then you start to
> understand current behaviour, but this is implementation details.
> 
> 
> > 
> > > So to have 
> > > desirable effect I have to create shared memory with shmget?
> > 
> > shmget behaves the same.
> > 
> Then I misinterpreted "Shared Policy" section from Lee's document.
> It seems that he states that for memory region created with shmget the
> policy is a property of a shared object and not of a process' address
> space.
> 
Man page states:
 Memory policy set for memory areas is shared by all threads of the
 process. Memory policy is also shared by other processes mapping the
 same memory using shmat(2) or mmap(2) from shmfs/hugetlbfs. It is not
 shared for disk backed file mappings right now although that may change
 in the future.
So what does this mean? If I set local policy for memory region in process
A it should be obeyed by memory access in process B?

--
			Gleb.

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