[Bug 129172] Experiments with mem=XXXM. Workaround.

[RichardNeill]

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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