huge pages, was where are the exit nodes gone?

[Scott Bennett]

     On Tue, 13 Apr 2010 11:04:36 +0200 Olaf Selke <olaf.selke at blutmagie.de>
wrote:
>Scott Bennett wrote:
>
>>      Either I forgot (probable) or you didn't mention before (less probable)
>> that you had moved it to a newer machine.  Whatever you're running it on,
>> superpages or LINUX's "huge" pages ought to speed tor up considerably by
>> drastically reducing TLB misses.  (I wasn't suggesting that you revert to
>> older hardware.  I was thinking that you were still running tor on the Xeon-
>> based machine.)
>
>I just setup hugepages (1024 pages a 2 MB) according this hint
>http://www.pythian.com/news/1326/performance-tuning-hugepages-in-linux/

     Very interesting article.  Thanks for the URL.  Of course, not being
a LINUX user, I have no idea what the acronyms for various LINUX kernel
features mean, and I have mercifully been free of any involvement with
Oracle for ~17 years, so ditto for the Oracle stuff. :-)
     One matter of concern, though, is the mention of a page size of 2 MB.
Intel x86-style CPUs offer a 2 MB page size *only* for instruction (a.k.a.
text) segments, not for data or stack segments, so I'm not sure what LINUX
is doing with that.  (See also the last line of the following bit of
output.)
>
>anonymizer2:~# echo 1024 > /proc/sys/vm/nr_hugepages
>anonymizer2:~# cat /proc/meminfo | grep -i hugepage
>HugePages_Total:     126
>HugePages_Free:      126
>HugePages_Rsvd:        0
>HugePages_Surp:        0
>Hugepagesize:       2048 kB
>
>Does tor process automagically take advantage from hugepages after
>restarting the process or has tor source code to be modified?
>
     Olaf, I honestly don't know.  I had not seen the page for which you
provided a URL, and it is more recent than what I had read about LINUX's
huge pages before.  Those older articles clearly stated that a program
had to reserve/designate its memory as huge pages *explicitly*, but it's
possible that usage is now more automatic.  However, part of the final
sentence in the article's summary section stands out to me:

	"If your database is running in LINUX *and has HugePages capability*
	[emphasis mine  --SB], there is no reason not to use it."

That suggests to me that the application (tor, in this case) must tell
the LINUX kernel which page size it wants for its memory.  Whether it
also has to specify address ranges explicitly to be so mapped, I haven't
the foggiest idea.  But even if the application does have to tell the
kernel something, it ought to be fairly trivial to add to tor's startup
code.  Start out by overestimating (assuming there is adequate real
memory on the system to play with) how much tor will need at its maximum
size, then decrease it, perhaps a bit at a time in successive recompilations,
until it only minimally exceeds tor's high-water mark.
[soapbox:  on]
     [Unless you have other applications also using that machine, this would
probably all be made so much easier by just trying out PC-BSD 8.0 because a
one-line entry in /boot/loader.conf would take care of superpages for you
automatically.  PC-BSD is the install-and-go version for both new users who
need to be able to use the system right away before learning much and casual
users who have no interest in learning much about FreeBSD.  This special
packaging of the system is designed to allow both groups, who might
otherwise find it beyond the effort they were willing or able to put into
it, to get its benefits.]
[soapbox:  off]
     Now that you've had tor running for a while, what does a
"cat /proc/meminfo | grep -i hugepage" show you?  Also, 126 such pages
equal 256 MB of memory.  Is that really enough to hold your entire tor
process when it's going full tilt?  I thought I had seen you post items
here in the past that said it was taking well over 1 GB and approaching
2 GB.


                                  Scott Bennett, Comm. ASMELG, CFIAG
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* Internet:       bennett at cs.niu.edu                              *
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