RE: [PATCH] turn off writable page tables

[Joe Bonasera <joe.bonasera@sun.com>]

xen-devel-request@lists.xensource.com wrote:

> 
> Message: 1
> Date: Wed, 26 Jul 2006 22:38:32 +0100
> From: "Ian Pratt" <m+Ian.Pratt@cl.cam.ac.uk>
> Subject: RE: [Xen-devel] [PATCH] turn off writable page tables
> To: "Andrew Theurer" <habanero@us.ibm.com>,	"Keir Fraser"
> 	<Keir.Fraser@cl.cam.ac.uk>
> Cc: Ian Pratt <m+Ian.Pratt@cl.cam.ac.uk>, Gerd Hoffmann
> 	<kraxel@suse.de>,	xen-devel@lists.xensource.com
> Message-ID:
> 	<A95E2296287EAD4EB592B5DEEFCE0E9D572247@liverpoolst.ad.cl.cam.ac.uk>
> Content-Type: text/plain;	charset="us-ascii"
> 
>> And it does make a difference in this case.  I now have a test program
>> which dirties a number of virtually contiguous pages then forks (it
> also
>> resets xen perf counters before fork and collects perf counters right
>> after fork), then records the elapsed time for the fork.  The
> difference
>> is quite amazing in this case.  For both writable and emulate, I ran
>> with a range of dirty pages, from 1280 to 128000.  The elapsed times
> for
>> fork a quite linear from small number to large number of dirty pages.
>> Below are the min and max:
>>
>>          1280 pages    128000 pages
>> wtpt:     813 usec      37552 usec
>> emulate: 3279 usec     283879 usec
> 
> Good, at least that suggests that the code works for the usage it was
> intended for. 
> 
>> So, in a -perfect-world- this works great.  Problem is most workloads
>> don't appear to have a vast percentage of entries that need to be
>> updated.   I'll go ahead and  expand this test to find out what the
>> threshold is to break even.  I'll also see if we can implement a
> batched
>> call in fork to update the parent -I hope this will show just as good
>> performance even when most entries need modification and even better
>> performance over wtpt with a low number of entries modified.
> 
> With license to make more invasive changes to core Linux mm it certainly
> should be possible to optimize this specific case with a batched update
> fairly easily. You could even go further an implement a 'make all PTEs
> in pagetable RO' hypercall, possibly including a copy to the child. This
> could potentially work better than current 'late pin', at least the
> validation would be incremental rather than in one big hit at the end. 
> 
> Ian

OpenSolaris could easily use the "make all PTEs in pagetable RO" hypercall.
But we don't copy in bulk to the child, so if you go down that path
please make the copy to child part optional.

Joe