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  #1 (permalink)  
Old 2nd January, 2009, 11:05 AM
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Custom kernel options

When compiling a new custom kernel, what are the benefits/drawbacks of changing the PREEMPT values and the kernel timer frequency values? PREEMPT value is by default "voluntary", there is much buzz about making it "preemptable"; also many reports of benefits from increasing the timer frequencies in the kernel config.

I use a non-tweaked custom kernel compiled for my machines, but is there any real benefit to playing with this?
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Old 2nd January, 2009, 08:58 PM
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Adjusting the PREEMPT value essentially adjusts how responsive the system is to certain requests.

Basically, with no preemption you are running the same way you always have (this is the default). When an application makes a call into the kernel, that call runs to completion. This means that no other process can get time slices until the kernel call is completed. This provides the greatest throughput in the system and the minimum overhead, but also means that the system cannot preempt a kernel call by a low-priority process in order to allow a higher-priority process to run. This isn't usually an issue for servers, because the primary consideration is usually maximizing total system performance. For desktops, this can be undesirable, because it can reduce the PERCIEVED responsiveness of the system from the desktop user perspective.

As a result, there is an option in the kernel to do preemptive kernel scheduling, which allows low-priority processes to be interrupted while executing kernel calls, if a higher-priority process needs to run. This reduces total throughput (because the kernel has to check more often to see if the current task needs to be suspended for a higher-priority task, and because task switches take time), but results in a more responsive system from a desktop-user perspective, particularly when the system is loaded.

However, the preemptive mode is not always responsive ENOUGH, particularly for embedded applications. As a result, a new mode has been added to allow for VOLUNTARY preemption. In this mode, even more checks are made in the kernel, and the application can even preempt ITSELF (think child processes or threads here).

In addition, there is an option to preempt the 'Big Kernel Lock' which is a dark and murky part of the kernel code that can negatively affect throughput. Unfortunately, there appear to not be any people around who fully understand all of the dependencies and consequences of removing the BKL. At this point, I gather that the kernel folks THINK it is pretty safe to remove, but they don't really KNOW, which means that under certain circumstances (mostly with arcane and seldom-used applications and device drivers) it is possible that removing the BKL may destabilize your system. However, it can also dramatically improve latencies (delays in responding to user inputs or system events).

Hope that answers your questions.
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Old 3rd January, 2009, 09:23 AM
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Well, I suppose it can't do any harm to compile a kernel or two with different settings as a test; at worst they aren't acceptable and the prior kernel can be fallen back upon.

I guess without a really technical, quantifiable way of testing it would really be a "user-experience" thing. Does it "feel" quicker on its feet when doing one's normal work? Then it is.
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Old 5th January, 2009, 06:36 AM
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My knowledge of kernels is limited and I don't care too much for any kernel later than the 2.6.23 series. I feel later kernel versions are too "interactive", which is fine if you mainly use multiple apps that don't require a lot of CPU power, but pretty much takes the bite out of single, CPU hungry apps.
I don't know of any benchmarks for preempt. For the most part I run my rigs with preempt disabled. The excpetion to this was with a single core HTPC. It required preempt to be enabled to avoid audio/video skip under certain circumstances. (trying to watch a movie in one room while someone was grabbing a file from another) . After installing a quad in that rig I turned preempt off again and all was well.

Which kernel and patchset are you running?
I've stuck with the 2.6.23-kamikaze5 for my main rig, it's been solid stable and fast for quite awhile, anything newer and my rig just doesn't do what I want it to do. That rig is on a 32bit install rather than 64 bit. For the apps I run 32 bit is still much much faster..

My server runs a 2.6.22 kernel. While that rig is built on Gentoo, package selection came from CentOS (or RHEL) for obvious stability reasons.

I don't build anything into the kernel that I don't need, I use modules whenever I can and will write a short script that loads the required module for apps that don't get used much, then link the script to the desktop or start menu.
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Old 5th January, 2009, 12:49 PM
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Quote:
Originally Posted by Gizmo View Post
Unfortunately, there appear to not be any people around who fully understand all of the dependencies and consequences of removing the BKL. At this point, I gather that the kernel folks THINK it is pretty safe to remove, but they don't really KNOW, which means that under certain circumstances (mostly with arcane and seldom-used applications and device drivers) it is possible that removing the BKL may destabilize your system. However, it can also dramatically improve latencies (delays in responding to user inputs or system events).

Hope that answers your questions.
Kind of funny how small things can have large effects. On new years I had three servers die, running different operating systems, in different datacenters on different sides of the USA all at once. Seams that it was a new years eve leap year correction bug. Something along the lines of the kernel locking the system time, so it can increment it by 2 seconds for the leap year, then requesting the time while it is locked causing a race condition.

Been subscribed to the linux kernel mailing list lkml.org and reading along with the bug report and resolution process. Its actually very interesting reading, nice to see how some of the best coders in the world do their thing and debate solutions before implementation.
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Old 5th January, 2009, 02:41 PM
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Quote:
Originally Posted by ccperf721p View Post
Which kernel and patchset are you running?
I've stuck with the 2.6.23-kamikaze5 for my main rig, it's been solid stable and fast for quite awhile, anything newer and my rig just doesn't do what I want it to do. That rig is on a 32bit install rather than 64 bit. For the apps I run 32 bit is still much much faster..
I was running the 2.6.23 until I upgraded to 'buntu 8.10; now it's on the 2.6.27 which is the kernel that ships with 8.10/64-bit. I had compiled a custom kernel on the former, but I'm keen to learn how to prune it back and keep it lean.

Quote:
Originally Posted by ccperf721p View Post
I don't build anything into the kernel that I don't need, I use modules whenever I can and will write a short script that loads the required module for apps that don't get used much, then link the script to the desktop or start menu.
This is exactly what I want to learn how to do correctly, how to determine what's really needed and what to actually build into the kernel, and how to *properly* create modules. Can you point me to a good reference? There are so many, and while I am willing to do the homework, it would be helpful to have a jumping-off point.

@dsio: 2 seconds=3 lost servers? Musta been some party...
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Old 5th January, 2009, 04:13 PM
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One of them actually had a main hard disk failure too, weeks prior that we hadn't noticed. It was a Xen server, so once it boots the xen hypervisor and the kernel it doesn't touch its hard disk, even for swap. So when it rebooted, "DISK BOOT FAILURE"
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Old 5th January, 2009, 07:25 PM
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Old 5th January, 2009, 07:29 PM
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Quote:
Originally Posted by ThunderRd View Post
This is exactly what I want to learn how to do correctly, how to determine what's really needed and what to actually build into the kernel, and how to *properly* create modules. Can you point me to a good reference? There are so many, and while I am willing to do the homework, it would be helpful to have a jumping-off point.
The modules you absolutely HAVE to build into the kernel are the ones you need to boot the system: chipset, storage subsystem (scsi, ide, sata, etc), and filesystem.

For example, AOA runs on a server that supports both SATA and SAS, as well as IDE. Our drives are attached to the SAS controller, so that driver is built into the kernel. The SATA and IDE drivers can be loaded as modules, because we don't boot off of those.

Our root file system is built on JFS, so that filesystem driver is built into the kernel. The other filesystems that we use (EXT3, CDFS, etc) are all built as modules.

To build a driver as a module rather than including it in the kernel, all you have to do is select the driver in the menu, and toggle the selection until it shows an 'M' instead of a '*' (I'm assuming you are using menuconfig).
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Old 6th January, 2009, 03:29 AM
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I can walk you through configuring the kernel over IM if you'd like. I'm going to load up Ubuntu 8.10 in a VM to make it easier. I believe everything is the same in Buntu as it is in Debian, which I am familiar with.
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Old 6th January, 2009, 07:45 AM
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Quote:
Originally Posted by ccperf721p View Post
I can walk you through configuring the kernel over IM if you'd like. I'm going to load up Ubuntu 8.10 in a VM to make it easier. I believe everything is the same in Buntu as it is in Debian, which I am familiar with.
Thanks, Cliff, I may take advantage of that offer. I'm not ready to get my head into it completely right now, but will be in a week or two. Too many other things on my plate ATM.

I'll do a bit more reading and then get back to you on that when I'm ready. Thanks
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Old 6th January, 2009, 07:11 PM
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Speaking from a direct experience of the man, Cliff is a excellent guide through the Arcane mysteries that surround the coming of the Penguin!

(Sorry, it's my religious studies back ground..":O}
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Old 7th January, 2009, 01:48 AM
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Quote:
Originally Posted by Daniel ~ View Post
Speaking from a direct experience of the man, Cliff is a excellent guide through the Arcane mysteries that surround the coming of the Penguin!

(Sorry, it's my religious studies back ground..":O}
I wish I had a little more time to dedicate to learning the mysteries of the penguin. Unfortunately I make my living in another field so I have to try to keep up with that first.

@TR.. When you say custom kernel, do you mean customizing (as in configuring) the ubuntu kernel to your needs or going with a different kernel altogether?

Leaning down the current kernel isn't a difficult chore to undertake. Going into the kernel hacking menu can lead to bugs. Going with a custom patchset is another matter.

2.6.23-kamikaze1 was called "Die Computer, die! die! die!" While the kernel was very fast, it had a few "quirks" as you can probably imagine. On rigs other than my sig rig, which is just for fun, I will usually choose a tried and true vanilla kernel and apply patches based on need.
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Old 7th January, 2009, 05:19 AM
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Quote:
2.6.23-kamikaze1 was called "Die Computer, die! die! die!" While the kernel was very fast, it had a few "quirks" as you can probably imagine.
I imagine even penguins some times just get bored out on the ice and want to see something blow-up real good!
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Old 7th January, 2009, 08:38 AM
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Quote:
Originally Posted by ccperf721p View Post
@TR.. When you say custom kernel, do you mean customizing (as in configuring) the ubuntu kernel to your needs or going with a different kernel altogether?

I will usually choose a tried and true vanilla kernel and apply patches based on need.
What I want to do is exactly that...using the shipped kernel and re-compiling it for what my box needs, thinning it out. I've done it before, and it worked fine, but I'm starting to understand that there were some other things I could have done to be more "aggressive" in the compiling, and get a more efficient result.

I'm learning and adjusting quite quickly to Linux; it's not terribly different from the UNIX I remember from way back [and much of it is identical] ...except it's 100% clearer to me now than it was then

After I'm fully comfortable with the kernel compiling process I'll do some experimental stuff on my home server [in pursuit of further knowledge]. Don't want to go there just yet, though.
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Old 10th January, 2009, 07:43 AM
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This is probably a bit off topic, but I'm curious as to whether any of you have tried a Linux RT kernel out. I've had to study the theory behind it all in recent years and I'm aware of the downsides to an RT kernel but out of curiosity I installed a 2.6.26 RT kernel for Fedora 10, build the 3d drivers and gave it a go.

Some of the results have been rather interesting.
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Old 10th January, 2009, 09:08 AM
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Quote:
Originally Posted by dsio View Post
...out of curiosity I installed a 2.6.26 RT kernel for Fedora 10...

Some of the results have been rather interesting.
How so? I'm interested because it actually is very related to my question in the OP regarding kernel preemption.
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Old 10th January, 2009, 10:45 AM
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Its hard to tell whether I'm imagining it or not, but the desktop feels very snappy. Like propperly fast. I'd expect that to some degree given how 'lazy' recent kernels have begun to feel but I didn't expect to notice it this much. The other interesting thing is that while playing world of warcraft on the RT kernel, the response time is incredible. It was almost laggy, the keyboard response, but now it feels noticeably faster. The part that surprised me was that network latency is also lower, while I was expecting the opposite.

The only quirk is that sometimes, and this is rather rare, but sometimes, a spellcast in WoW will be really slow, like 3 seconds rather than 0.001 seconds. Also that sometimes there is some odd behavior, particularly from desktop menus.

All of this falls into the subjective rather than scientific category and it could be the placebo effect.

There are kernel-rt packages for Fedora 10 and also for Ubuntu (I think its here https://wiki.ubuntu.com/RealTime/Hardy).

All rather interesting.
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Old 10th January, 2009, 02:21 PM
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Since I've been experimenting with kernels I will toss one together with RT when I have time and see if I get the same results. It is available as an apt-get package in the Ibex repos, so I could install it from there without even compiling, just to try it out.

Question about compiling:
After using linux-1.2.3.tar.bz2 to compile linux-image-1.2.3.deb, and then the .deb is installed using dpkg, do the files residing in /usr/src have to remain available to the OS for any reason? Specifically, the .deb file itself, and teh resulting /usr/src/linux-1.2.3 directory[which is sizable], and the symlink called simply "linux"?

Or can they be archived on another drive or deleted completely?

@ccperf:
I had some time today, built a new kernel using Zen kernel source; was able to get the kernel size to 13MB after three attempts [the first two were unbootable on account of I lost my mind somewhere in menuconfig]. I don't think I'll do too much better than that, perhaps a couple MB more if I really use a scapel, but I think that is quite satisfactory for now.

On that subject, after building an unbootable kernel, is it possible, rather than starting all over, to simply enter the linux directory, run 'make menuconfig/xconfig/whatever', import the previous .config file, change the options, and then re-compile on top of the previous compile? Or is that a recipe for disaster?
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Last edited by ThunderRd; 10th January, 2009 at 02:29 PM.
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Old 10th January, 2009, 03:02 PM
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Join Date: April 2005
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The /usr/src/linux < current kernel version > has to stay, everything else can be removed.

You can use your old config or any config from the same kernel as a starting point for building the kernel.

Zen sources are pretty good. Same guy that put together kamikaze-sources.

I recently had to move to a 2.6.27 zen kernel. Seems my favorite 2.6.23 kernel and core i7 do not agree with each other. 2.6.23 made it feel like I was using a P2. The 8 logical cores of the i7 eliminate the short-comings I had with the later kernels..
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