A useful test/workaround is provided by mem=XXX. This allowed me to experiment.
Here are the results.
In all cases, the system was booted in recovery mode (for speed of testing),
appending mem=XXXM to the kernel command-line. All 4 DIMMs (8GB in total)
were always present.
Results:
boot param result of free-m (total) performance
-------------- ---------------------------- ----------------
mem=2048M 2015 normal
mem=6144M 5477 normal
mem=8192M 7497 normal
mem=8792M 8002 100x slow #measured
by timing: for ((i=0;i<10000;i++)); do echo $i > /dev/null ;done
mem=10000M 100x slow
[none] 100x slow
I then did a binary search to find the optimum. Where bootup was very
slow, it was terminated with Alt-SysRQ-[RSEIUB] before completion, so
"free -m" was not measured. (Unfortunately, it doesn't work to use
"init=/bin/bash" to speed up the test cycle - why not?)
boot param free -m performance
-------------- ---------
----------------
mem=8500M slow
mem=8300M 7604 normal
mem=8400M slow
mem=8350M slow
mem=8325M slow
mem=8315M 7618 normal
mem=8320M slow
mem=8318M 7621 normal
mem=8319M slow
I also found these other links which might be relevant:
http://www.hostingforum.ca/172078-slow-kernel-when-memory-8g.html
http://lkml.org/lkml/2007/5/30/79
Lastly, looking at the difference between real, and reported memory:
(always with 8GB of physical RAM), from above data
mem= free -m difference
-------- --------- ------------
2048 2015 33
6144 5477 667
8192 7497 695
8792 8002 790
8300 7604 696
8315 7618 697
=> No very clear pattern.
Anyway, the workaround, for now is to boot with "mem=8318M", which actually
provides 7621MB of RAM (i.e. wasting 1171 MB).
--
kernel performance is *very* slow with 8GB RAM on AMD64. 6GB is fine. kernel
2.6.22-8 x86_64
https://bugs.launchpad.net/bugs/129172
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