kernel memory checks on boot vs. boot time

Bjoern A. Zeeb bz at
Tue Mar 22 17:31:18 UTC 2011


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


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?


Bjoern A. Zeeb                                 You have to have visions!
          Stop bit received. Insert coin for new address family.

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