bugThe GNU Hurd - Bugs: bug #18502, Suboptimal cache configuration for...

 
 

bug #18502: Suboptimal cache configuration for IA-32 processors

Submitter:  -Deleted Account- <syn>
Submitted:  Sun 10 Dec 2006 07:48:29 PM UTC
   
 
Category:  GNU Mach Severity:  3 - Normal
Priority:  5 - Normal Item Group:  Hardware Support
Status:  Fixed Privacy:  Public
Assigned to:  None Originator Name: 
Open/Closed:  Closed Reproducibility:  Every Time
Size (loc):  None Planned Release:  None
Effort:  0.00
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Sat 23 Apr 2011 01:35:00 PM UTC, comment #3: 

There is no real basic reason why caching should hit us, so I have commited it.

Samuel Thibault <sthibaul>
Group administrator
Thu 14 Dec 2006 02:03:10 AM UTC, comment #2: 

OK, I built gnumach with the following in model_dep.c:

set_cr0((get_cr0() | CR0_PG | CR0_WP) & ~(CR0_CD | CR0_NW));

And I got worse performance:

0m0.040s 0m0.130s
0m0.000s 0m0.000s

Booted back to normal gnumach:

0m0.010s 0m0.020s
0m0.000s 0m0.000s

That is just from running times on a little C program that counts to 1000000000, so it's obviously not scientific.

Barry deFreese <bddebian>
Sun 10 Dec 2006 11:17:47 PM UTC, comment #1: 

I tried clearing those two bits, in i386/i386at/model_dep.c, i386at_init(), by altering the call to set_cr0(). GNU Mach then crashed almost immediately during bootstrap. I didn't investigate further.

-Deleted Account- <syn>
Sun 10 Dec 2006 07:48:29 PM UTC, original submission:  

GNU Mach operates with a low-performance caching mode. When running, Control Register 0 (CR0) value is 0xe001001b. The CD (cache disable) and NW (Not Write-back/through) bits are set. This doesn't completely disable cache usage (at least on modern processors), but makes the processor use a suboptimal mode. See IA-32 Intel Architecture Software Developer's Manual Volume 3: System Programming Guide, 10.5.1 Cache Control Registers and Bits, Figure 10-5.

-Deleted Account- <syn>

 

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    Follow 2 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2011-04-23 sthibaul StatusNone Fixed
        Open/ClosedOpen Closed

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