Very Large "zero" slab expected?

[Sonny Rao]

On Wed, Feb 02, 2005 at 10:39:02AM +1100, Paul Mackerras wrote:
> Sonny Rao writes:
> 
> > Hi, I'm running a fairly heavy database workload using DB/2 on a 4-way
> > 720 ontop of ext3 trying to replicate a separate VM performance issue.
> > 
> > During and after the run, I see a massive about of slab memory being
> > used by the "zero" cache, which appears to a be a ppc64 specific
> > cache for pmd's and pgd's.  
> > 
> > Basically, about 1GB out of my total of 8GB on the system is being
> > consumed by this.  Is this expected ?  It seems excessive
> > considering my mapped memory is only about 2GB during the run.
> > 
> > The DB/2 buffer pool was set to 4GB and none of it used hugh pages.
> 
> How many DB/2 processes had the buffer pool mapped?  It was probably
> 100 or more, I would guess.  The zero slab cache is used for pte pages
> as well as pmds and pgds, and each process will have a separate page
> table tree, which will occupy about 1/500th or so of the amount of
> mapped memory, per process.  So if you had ~125 processes the page
> table trees would occupy about 1GB for a 4GB buffer pool.
> 
> The solution is to implement shared page tables.  Some people here in
> the LTC are looking at possibly doing that, but getting it accepted
> into mainline could be tricky.


By the way, the name "zero" slab is not very descriptive, some of the
PPC64 developers here in Austin didn't even know what it was off hand.  

You might consider renaming it to avoid confusion.

Thanks,

Sonny