turn off writable page tables

At OLS I gave a talk on some of the Xen scalability inhibitors, and one of these was writable page tables. We went over why the feature does not scale, but just as important, we found that the uniprocessor case does not provide any advantage either. These tests were done on x86_64, so I wanted to run the 1-way test on 32 bit to show the same problem. So, I have run with writable PTs and with emulation forced on for several benchmarks:

on Xeon MP processor, uniprocessor dom0 kernel, pae=y:

benchmark c/s 10729 force_emulate
------------------------ --------- -------------

lmbench fork+exit:       469.5833  470.3913   usec, lower is better
lmbench fork+execve:     1241.0000 1225.7778  usec, lower is better
lmbench fork+/sbin/bash: 12190.000 12119.000  usec, lower is better
dbench 3.03              186.354   191.278    MB/sec
reaim_aim9               1890.01   2055.97    jobs/min
reaim_compute            2538.75   2522.90    jobs/min
reaim_dbase              3852.14   3739.38    jobs/min
reaim_fserver            4437.93   4389.71    jobs/min
reaim_shared             2365.85   2362.97    jobs/min
SPEC SDET                4315.91   4312.02    scripts/hr

These are all within the noise level (some slightly better, some slightly worse for emulate). There really isn't much of difference here. I'd like to propose turning on the emulate path all the time in xen.

-Andrew Theurer

Applies to c/s 10729

diff -Naurp xen-unstable.hg-10729/xen/arch/x86/mm.c xen-unstable.hg-10729-emulate/xen/arch/x86/mm.c
@@ -3582,7 +3582,7 @@ int ptwr_do_page_fault(struct domain *d,
         return 0;
     }
 
-#if 0 /* Leave this in as useful for debugging */ 
+#if 1 /* Leave this in as useful for debugging */ 
     goto emulate; 
 #endif
 

_______________________________________________
Xen-devel mailing list
[email protected]
http://lists.xensource.com/xen-devel

Andrew, is this something for 3.0.3/Fedora6/RHEL5 consideration, or post-3.0.3?

thanks,
Nivedita

I think it could go either way. There should be no "risk" using emulate over writable PT when it comes to stability, etc.

-Andrew