Re: What to expect with the 2.6 VM

[William Lee Irwin III <wli@holomorphy.com>]

On Thu, 3 Jul 2003, Andrea Arcangeli wrote:
>> even if you don't use largepages as you should, the ram cost of the pte
>> is nothing on 64bit archs, all you care about is to use all the mhz and
>> tlb entries of the cpu.

On Thu, Jul 03, 2003 at 09:06:32AM -0400, Rik van Riel wrote:
> That depends on the number of Oracle processes you have.
> Say that page tables need 0.1% of the space of the virtual
> space they map.  With 1000 Oracle users you'd end up needing
> as much memory in page tables as your shm segment is large.
> Of course, in this situation either the application should
> use large pages or the kernel should simply reclaim the
> page tables (possible while holding the mmap_sem for write).

No, it is not true that pagetable space can be wantonly wasted
on 64-bit.

Try mmap()'ing something sufficiently huge and accessing on average
every PAGE_SIZE'th virtual page, in a single-threaded single process.
e.g. various indexing schemes might do this. This is 1 pagetable page
per page of data (worse if shared), which blows major goats.

There's a reason why those things use inverted pagetables... at any
rate, compacting virtualspace with remap_file_pages() solves it too.

Large pages won't help, since the data isn't contiguous.


-- wli
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