Re: [PATCH] Physical Memory Management [0/1]

Andi Kleen
2009-05-15T11:21:19Z

On Fri, May 15, 2009 at 12:47:23PM +0200, MichaA? Nazarewicz wrote:
> On Fri, 15 May 2009 12:18:11 +0200, Andi Kleen wrote:
> > That's not correct, support for multiple huge page sizes was recently
> > added. The interface is a bit clumpsy admittedly, but it's there.
>
> I'll have to look into that further then. Having said that, I cannot
> create a huge page SysV shared memory segment with pages of specified
> size, can I?

sysv shared memory supports huge pages, but there is currently
no interface to specify the intended page size, you always
get the default.

>
> > However for non fragmentation purposes you probably don't
> > want too many different sizes anyways, the more sizes, the worse
> > the fragmentation. Ideal is only a single size.
>
> Unfortunately, sizes may very from several KiBs to a few MiBs.

Then your approach will likely not be reliable.

> On the other hand, only a handful of apps will use PMM in our system
> and at most two or three will be run at the same time so hopefully
> fragmentation won't be so bad. But yes, I admit it is a concern.

Such tight restrictions might work for you, but for mainline Linux the quality
standards are higher.

> > As Peter et.al. explained earlier varying buffer sizes don't work
> > anyways.
>
> Either I missed something or Peter and Adrew only pointed the problem
> we all seem to agree exists: a problem of fragmentation.

Multiple buffer sizes lead to fragmentation.

-Andi
--
ak@linux.intel.com -- Speaking for myself only.