[26-devel] v2.6 performance slowdown on MPC8xx: Measuring TLB cache misses

Marcelo Tosatti marcelo.tosatti at cyclades.com
Mon Apr 25 02:55:20 EST 2005

Hi Dan, Joakim,

On Sat, Apr 23, 2005 at 08:00:39PM -0400, Dan Malek wrote:
> On Apr 23, 2005, at 7:51 PM, Joakim Tjernlund wrote:
> >hmm, I have more than 24MB of memory and I can run CONFIG_PIN_TLB just
> >fine with modules off in kernel 2.4. Havn't tried 2.6 yet.
> Doh.  Oh, I see.  We only do the optimization for the instruction 
> misses.
> I'll have to take a closer look at Marcelo's 2.6 tests.

The PIN TLB entry option does not make much difference in my tests, 
never did.

Who wrote the code? Are there results which indicate a performance gain
from TLB pinning on 8xx? If so, where are such results? 

One problem that I've noted is that initial_mmu sets {I,D}TLB index
to be 27 (11100). 

MI_RSV4I protects TLB's 27...31.

Given that both {I,D}TLB INDEX's are _decreased_ on each update, it seems
to me that initial_mmu should set {I,D}TLB INDEX to 31, which will then 
decrease down to 27 after 4 TLB's are created.  

Another question that comes to mind is why initial_mmu does create 
additional 8Meg TLB entries for D-cache but not for I-cache: 

        /* Map two more 8M kernel data pages.

I'll do some more CONFIG_PIN_TLB tests this week...

--- head_8xx.S.orig2	2005-04-24 17:55:59.000000000 -0300
+++ head_8xx.S	2005-04-24 17:57:44.000000000 -0300
@@ -697,7 +697,7 @@
 	tlbia			/* Invalidate all TLB entries */
 	lis	r8, MI_RSV4I at h
-	ori	r8, r8, 0x1c00
+	ori	r8, r8, 0x1f00
 	li	r8, 0
@@ -705,7 +705,7 @@
 	lis	r10, (MD_RSV4I | MD_RESETVAL)@h
-	ori	r10, r10, 0x1c00
+	ori	r10, r10, 0x1f00
 	mr	r8, r10
 	lis	r10, MD_RESETVAL at h

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