Dan Malek [off-list ref] wrote:

On Jul 26, 2004, at 10:27 PM, Linh Dang wrote:

quoted

What do you mean by the above sentence?

Exactly what I said.  If you are concerned about the difference in
performance between a BAT mapped space and page tables,
there are many other kernel behaviors that are going to cause
trouble for your software.

quoted

- 200MB would need 51200 ptes. that means doubling the current number
of ptes on my system.

Doubling?  I don't think so.  How did you measure what you are

# cat /proc/ppc_htab
PTE Hash Table Information
Size            : 256Kb
Buckets         : 4096
Address         : c02c0000
Entries         : 32768
User ptes       : 326
Kernel ptes     : 989
Percent full    : 4%
Reloads         : 29288
Preloads        : 17175
Searches        : 1791
Overflows       : 0
Evicts          : 0
Non-error misses: 24468
Error misses    : 0


That's about 1315 PTEs (before the applications start.)

currently using?  The 51200 PTEs really isn't a lot.  Mapping huge

just 38 times what currently on my system.

linear spaces with PTEs is actually quite efficient.  Small VM spaces
with holes are the killer.  Do you actually touch every byte within
that 200 MB space?

When the system is working full-steam it's pretty close too that.

quoted

- If using block mapping doesn't help that much then why would X make
all the effort to grab MTRRs on X86?

I dunno.  I've never done any performance or feature analysis of x86
page
tables to discuss this.

quoted

- why would the kernel use BATs to map its memory?

It's convenient for some areas of memory.  Makes it trivial to write
some forms of IO mapping functions.  We can set up some early static
memory maps for kernel initialization.  Mainly, we don't pollute the
TLBs
during short interrupt or system calls, allowing the user applications
to
run without having to reload the TLB after such events.  Even though
the kernel may use BATs, it still maps everything with page tables and
makes extensive use of them for various memory mapping requirements.

I though the kernel (2.6) only use pages for vmalloc.

--
Linh Dang

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