kernel memory checks on boot vs. boot time

[George Neville-Neil]

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On Mar 22, 2011, at 13:30 , Bjoern A. Zeeb wrote:

> Hi,
> 
> as part of the i386/pc98/amd64 boot process we are doing some basic
> memory testing, mapping pages and running a couple of pattern
> write/read tests on the first bytes (see getmemsize() implmentations).
> 
> Depending on the features enabled and boot -v or not you may notice
> it as "nothing happens" booting from loader, after any of these
> possible lines:
> 	GDB: no debug ports present
> 	KDB: debugger backends: ddb
> 	KDB: current backend: ddb
> 	SMAP type=...
> but before the Copyright message.
> 
> With the growing number of memory this can lead to a significant
> fraction of kernel startup time on amd64 (~40s delays observed with
> 96G of RAM).  Looping over the pages, but not mapping them and not
> running the pattern tests reduces this significantly (to single digit
> numbers of seconds).
> 
> As a first step I'd like to discuss how worth the actual memory tests
> are these days, to figure out a sensible default.
> 
> Not wanting to remove them but maybe make more use of them in the
> future (as we do not report any problems we find currently) I'd suggest
> to introduce a tunable to disable/enable them, say
> 
> 	hw.run_memtest
> 
> with the following values:
> 
> 	0	do not map the page and do not run the pattern tests
> 	1	do run the pattern test on the beginning of the page
> 		(current default).
> and maybe add
> 	2	run the pattern tests on the entire pages?
> 
> I would further suggest to add a printf independently of boot -v
> there, so that the user who would wait, will know what's (not) going on.
> Something along the lines of:
> 	"Testing physical address space (%s)."
> 	0       "skipping extra pattern tests"
> 	1       "pattern tests on beginning of each page"
> 	2       "pattern tests on entire pages"
> 
> 
> If this is something that makes sense, I'd suggest to factor things
> out to sys/x86 and would provide a patch for further discussion and
> improvements (like error reporting, etc).
> 
> Comments?  Suggestions?

I do not know how effective our memory tests on are on modern systems.

I do think that having a tunable is a good idea for people who want
faster boots.

Best,
George

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