On Tue, Mar 28, 2006 at 02:56:40PM -0600, Robin Holt wrote:
> On Tue, Mar 28, 2006 at 11:16:19AM -0800, Dave Hansen wrote:
> > Could you explain a bit how this works on ia64? I know about the
> > vmem_map. Is the time spent on filling TLB entries when you hit a
> > 'struct page' that isn't backed by real memory?
>
> Time is wasted trying to fill the TLB entry for the vmem_map. When it
> fails, we show_mem() advances to the next page which repeats the sequence.
> Jack had thrown out a couple suggestions. One was essentially what
> you proposed below. The other was advance i to point the next page
> of pfns. He frowned when saying the second, but I don't recall exactly
> why he frowned.
Advancing to the next page will be considerably faster but I wonder if
it is fast enough.
There are huge gaps in the virtual vmem_map. On shub2, for example, it
is possible to have 180GB of unpopulated memory in the holes
between memory banks on a node (mode=0).
Assuming 56 bytes per struct_page, that gives:
- 180GB = 11M pages
- 38000 pages of struct_page entries
- 38000 TLB faults to scan the holes in a node
That is a lot of tlbmisses to scan a node. Multiply by 512 to
get the number of faults to scan a full 512n system.
My gut feeling is that is not good enough.
>
> > Maybe something like this in your show_mem():
> >
> > for_each_pgdat(pgdat) {
> > ...
> > for(i = 0; i < pgdat->node_spanned_pages; i++) {
> > struct page *page;
> > if (pfn_valid(pgdat->node_start_pfn + i))
> > page = pfn_to_page(pgdat->node_start_pfn + i);
> > else
> > - continue;
> > + /* -1 to offset i++ */
> > + pfn += hole_nr_pages(pfn) - 1;
> >
--
Thanks
Jack Steiner (steiner@xxxxxxx) 651-683-5302
Principal Engineer SGI - Silicon Graphics, Inc.
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