[gentoo-amd64] Re: tmpfs help

[Duncan <1i5t5.duncan@cox.net>]

Beso <givemesugarr@gmail.com> posted
d257c3560802131127g51bccaf6s920e73d1f2416766@mail.gmail.com, excerpted
below, on  Wed, 13 Feb 2008 19:27:44 +0000:

> i'll try out duncan's speedups for shm and but for the dev one i don't
> use baselayout 2 and i'm still with the 1st version, since i don't feel
> like upgrading to it yet. but i'd like to know some more about what are
> the improvements of the second version.

Well, among other things it's possible to /really/ do parallel startup 
now.  The option was there before, but it didn't parallelize that much.  
Of course, if you have any startup scripts running that the dependencies 
aren't straight on, that might be hidden now, but will very likely show 
itself once the system actually does try things in parallel.  (For those 
who prefer traditional serial startup, that remains the safer default.)

Startup's also much faster as certain parts are written in C now, instead 
of as scripts.  Of course, the various service scripts remain just that, 
scripts.

With baselayout-1, the very early core scripts, clock, modules, lvm, 
raid, etc, were actually ordered based on a list rather than on their 
normal Gentoo dependencies (before, after, uses, needs, etc).  That was 
because the dependency resolver didn't work quite right that early on.  
That has now been fixed, and all scripts including the early stuff like 
clock, start in the order the dependencies would indicate, not based on 
an arbitrary list.

Various settings are in more logical/traditional locations with 
baselayout 2.  An example is the /dev filesystem mounted if you have udev 
active.  Previously, its configuration was in one or more of the 
baselayout config files (probably /etc/conf.d/rc but that was quite 
awhile ago here, and I've forgotten the details so can't be sure).  Now, 
the setting in /etc/fstab for that filesystem is honored, as one might 
ordinarily expect for any filesystem.

Former addons like lvm and raid now have their own initscripts, just as 
any other boot service.

> 5. as for the raid stuff i cannot do it since i've only got one disk.
> i'll try to see what happens with swap set to 100.

For a single disk, it's possible you'll actually want it set the other 
way, toward 0 from 60, especially if you are annoyed at the time it takes 
to swap programs back in after big compiles or a night's system scan for 
slocate (I forgot what the scanner is called, as I don't have slocate 
installed here so don't run the database updater/scanner).  On a system 
such as mine with swap roughly twice as fast as the main filesystems, 
however, keeping cache and swapping out apps makes much more sense, since 
it's faster to read back in the apps from swap than it is to reload the 
data I'd dump from cache to keep the apps in memory.  I actually don't 
even notice swap usage unless I happen to look at ksysguard, unless it's 
swapping over a gig, which doesn't happen too often with 8 gigs RAM.  
Still, it's quite common to have a quarter to three quarter gig of 
swapped out apps since I've set swappiness to 100, thereby telling the 
kernel to keep cache if at all possible, and I routinely do -j12 
compiles, often several at a time even, so several gigs of tmpfs plus 
several gigs of gcc instances in memory, thereby forcing minor swapping 
even with 8 gig RAM, isn't unusual.  (Of course, I use emerge --pretend 
to ensure the packages I'm emerging in parallel don't conflict with or 
depend on each other, so the merges remain parallel.)

> 6. if i use some
> other programs while compiling into tmpfs bigones i need to nice the
> process or i'll get some misbehaviour from the other programs.

Here's another hint.  Consider setting PORTAGE_NICENESS=19 in make.conf.  
Not only does it stop the hogging, the system then considers it batch 
priority and gives it longer timeslices.  Thus, counter-intuitively for 
lowering the priority, a merge can actually take less real clock time, 
because the CPU is spending less time shuffling processes around and more 
time actually doing work.  Of course, if you run folding@home or the 
like, you'll either want to turn that off when doing this, or set portage 
to 18 or better instead of 19, so it's not competing directly with your 
idle time client.

With kernel 2.6.24, there's also a new scheduling option that groups by 
user.  By default, root gets 2048 while other users get 1024, half the 
share of root.  If you have portage set to compile with userprivs, it'll 
be using the portage user, with its own 1024 default setting, but if it's 
using root privs, it'll be getting root's default 2048 share.  
Particularly if you use a -j setting >2 jobs per cpu/core, you may wish 
to tweak this some.  I use FEATURES=userpriv and userfetch, so most of 
the work gets done as the portage user altho the actual qmerge step is 
obviously done as root, and MAKEOPTS="-j20 -l12" (twenty jobs, but don't 
start new ones if the load is above 12) and my load average runs pretty 
close, actually about 12.5-13 when emerging stuff.  With four cores (two 
CPUs times two cores each), that's a load average of just over three jobs 
per core.  As mentioned, I have PORTAGE_NICENESS=19.  Given all that, I 
find increasing my regular user to 2048 works about the best, keeping X 
and my various scrollers updating smoothly, amarok playing smoothly (it 
normally does) and updating its voiceprint almost smoothly (it doesn't, 
if I don't adjust the user share to 2048), etc.

This setting can be found under /sys/kernel/uids/<uid>/cpushare.  Read it 
to see what a user's share is, write it to write a new share for that 
user.  (/etc/passwd lists the user/uid correspondence.)

In ordered to have these files and this feature, you'll need to have Fair 
group CPU scheduling (FAIR_GROUP_SCHED), under General setup, and then 
its suboption, Basis for grouping tasks, set to user id 
(FAIR_USER_SCHED).  Unfortunately, the files appear as a user logs in (or 
is invoked by the system, for daemon users) and disappear as they logout, 
so it's not as simple as setting an initscript to set these up at boot 
and forgetting about it.  There's a sample script available that's 
supposed to automate setting and resetting these shares based on kernel 
events, but I couldn't get it to work last time I tried it, which was 
during the rcs I should say, so it's possible it wasn't all working just 
right, yet.  However, altering the share files manually works, and isn't 
too bad for simple changes, as long as you don't go logging in and out a 
lot, thus losing the file and the changes made to it every time there's 
nothing running as a particular user any longer.

Another setting that may be useful is the kernel's I/O scheduler.  That's 
configured under Block layer, I/O Schedulers.  CFQ (IOSCHED_CFQ) is what 
I choose.  Among other things, it automatically prioritizes I/O requests 
based on CPU job priority (altho less granularly, as there's only a 
handful of I/O priority levels compared to the 40 CPU priority levels).  
You can tweak priorities further if desired, but this seems a great 
default and is something deadline definitely doesn't have and I don't 
believe anticipatory has either.  With memory as tight as a gig, having 
the ability to batch-priority schedule i/o along with CPU is a very good 
thing, particularly when PORTAGE_NICENESS is set.  Due to the nature of 
disk I/O, it's not going to stop the worst thrashing entirely, but it 
should significantly lessen its impact, and at less than worst case, it 
will likely make the system significantly more workable than it might be 
otherwise under equivalent load.

Of course, YMMV, but I'd look at those, anyway.  It could further 
increase efficiency.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman

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