The Linux Process Model

This month, we begin looking at Linux internals. We will travel the innards of the Linux kernels of the 2.0.x, 2.2.x and the new 2.4.x series. Although many articles are written every week on how best to use Linux, very few have reviewed the internals of the kernel. Why is it necessary to know how the kernel works?

For one thing, understanding your kernel better will enable you to prevent problems before they occur. If you are using Linux as a server, most problems will start to appear under stress. This is exactly when it becomes essential to know your way around the kernel to assess the nature of the problems.

If you ever need to check back with the kernel source, you can either install the source from your distribution's CD or go to http://lxr.linux.no/source/ to navigate through all the source code on-line.

UNIX systems have a fundamental building block: the process, including the thread and lightweight processes. Under Linux, the process model has evolved considerably with each new version.

The fundamental data structure within the kernel controlling all processes is the process structure, which grows and shrinks dynamically as processes are forked and finished or killed.

The process structure (called task_struct in the kernel source code) is about 1KB in size. You can get the exact size with this program:

#define __KERNEL__
#include <linux/sched.h>
main()
{
printf("sizeof(struct task_struct) - %d\n",
   sizeof(struct task_struct));
}

On Intel 386 machines, it is exactly 960 bytes. Please note, however, that unlike other UNIX systems, this process structure does not occupy space in the true sense of the word.

Since 2.2.x, the task_struct is allocated at the bottom of the kernel stack. We can overlap the task_struct on the kernel stack because the task_struct is a per-task structure exactly as the kernel stack.

The kernel stack has a fixed size of 8192 bytes on the Intel x86. If the kernel will recurse on the stack for 8192-960=7232 bytes, then the task_struct will be overwritten and therefore corrupted, causing the kernel to crash.

Basically, the kernel decreases the size of the usable kernel stack to around 7232 bytes by allocating the task structure at the bottom of the stack. It is done this way, because 7KB are more than enough for the kernel stack and the rest is used for the task_struct. These are the advantages of this order:

Once Linux is in kernel mode, you can get the address of the task_struct at any time with this very fast pseudo-code:

task_struct = (struct task_struct *) STACK_POINTER & 0xffffe000;

/* cut-and-pasted from
linux/include/asm-i386/current.h */
static inline struct task_struct * get_current(void)
{
        struct task_struct *current;
        __asm__(-andl %%esp,%0;
        -:-=- (current) : "0" (~8191UL));
        return current;
        }

The layout of the x86 kernel stack looks like this:

----- 0xXXXX0000 (bottom of the stack and address
                 of the task struct)
TASK_STRUCT
----- 0xXXXX03C0 (last byte usable from the kernel
                  as real kernel stack)
KERNEL_STACK
----- 0xXXXX2000 (top of the stack, first byte
                  used as kernel stack)

Note that today, the size of the task_struct is exactly 960 bytes. It is going to change across kernel revisions, because every variable removed or inserted to the task_struct will change the size. In turn, the upper limit of the kernel stack will change with the size of task_struct.

The memory for the process data structure is allocated dynamically during execution of the Linux kernel. More precisely, the kernel doesn't allocate the task_struct at all, only the two-pages-wide kernel stack of which task_struct will be a part.

In many UNIX systems, there is a maximum processes parameter for the kernel. In commercial operating systems like Solaris, it is a self-tuned parameter. In other words, it adjusts according to the amount of RAM found at boot time. However, in Solaris, you can still adjust this parameter in /etc/system.

In Linux 2.3.x (and in the future, 2.4.0), it is a run-time tunable parameter as well. On 2.2.x, it's a compile-time tunable parameter. To change it in 2.2.x, you need to change the NR_TASKS preprocessor define in Linux/include/linux/tasks.h:

#define NR_TASKS 512 /* On x86 Max 4092 or 4090
                        with APM configured. */

Increase this number up to 4090 to increase the maximum limit of concurrent tasks.

In 2.3.x, it is a tunable parameter which defaults to size-of-memory-in-the-system / kernel-stack-size / 2. Suppose you have 512MB of RAM; then, the default upper limit of available processes will be 512*1024*1024 / 8192 / 2 = 32768. Now, 32768 processes might sound like a lot, but for an enterprise-wide Linux server with a database and many connections from a LAN or the Internet, it is a very reasonable number. I have personally seen UNIX boxes with a higher number of active processes. It might make sense to adjust this parameter in your installation. In 2.3.x, you can also increase the maximum number of tasks via a sysctl at runtime. Suppose the administrator wants to increase the number of concurrent tasks to 40,000. He will have to do only this (as root):

echo 40000 > /proc/sys/kernel/threads-max

In the last 10 years or so, there has been a general move from heavyweight processes to a threaded model. The reason is clear: the creation and maintenance of a full process with its own address space takes up a lot of time in terms of milliseconds. Threads run within the same address space as the parent process, and therefore require much less time in creation.

What's the difference between process and thread under Linux? And, more importantly, what is the difference from a scheduler point of view? In short—nothing.

The only worthwhile difference between a thread and a process is that threads share the same address space completely. All the threads run in the same address space, so a context switch is basically just a jump from one code location to another.

A simple check to avoid the TLB (translation lookaside buffer, the mechanism within the CPU that translates virtual memory addresses to actual RAM addresses) flush and the memory manager context switch is this:

/* cut from linux/arch/i386/kernel/process.c */
/* Re-load page tables */
{
        unsigned long new_cr3 = next->tss.cr3;
        if (new_cr3 !=3D prev->tss.cr3)
        asm volatile("movl %0,%%cr3": :"r" (new_cr3));
        }

The above check is in the core of the Linux kernel context switch. It simply checks that the page-directory address of the current process and the one of the to-be-scheduled process are the same. If they are, then they share the same address space (i.e., they are two threads), and nothing will be written to the %%cr3 register, which would cause the user-space page tables to be invalidated. That is, putting any value into the %%cr3 register automatically invalidates the TLB; in fact, this is actually how you force a TLB flush. Since two tasks in the same address space never switch the address space, the TLB will never be invalidated.

With the above two-line check, Linux makes a distinction between a kernel-process switch and a kernel-thread switch. This is the only noteworthy difference.

Since there is no difference at all between threads and processes, the Linux scheduler is very clean code. Only a few places related to signal handling make a distinction between threads and processes.

In Solaris, the process is greatly disadvantaged compared to the thread and lightweight processes (LWP). Here is a measurement I did on my Solaris server, an Ultra 2 desktop, 167MHz processor, running Solaris 2.6:

hirame> ftime
Completed 100 forks
Avg Fork Time: 1.137 milliseconds
hirame> ttime
Completed 100 Thread Creates
Avg Thread Time: 0.017 milliseconds

I executed 100 forks and measured the time elapsed. As you can see, the average fork took 1.137 milliseconds, while the average thread create took .017 milliseconds (17 microseconds). In this example, thread creates were about 67 times faster. Also, my test case for threads did not include flags in the thread create call to tell the kernel to create a new LWP with the thread and bind the thread to the LWP. This would have added additional weight to the call, bringing it closer to the fork time.

Even if LWP creation closes the gap in creation times between processes (forks) and threads, user threads still offer advantages in resource utilization and scheduling.

Of course, the Linux SMP (and even uniprocessor) scheduler is clever enough to optimize the scheduling of the threads on the same CPU. This happens because by rescheduling a thread, there won't be a TLB flush and basically no context switch at all—the virtual memory addressing won't change. A thread switch is very lightweight compared to a process switch, and the scheduler is aware of that. The only things Linux does while switching between two threads (not in strict order) are:

Nothing more is done. The TLB is not touched, and the address space and all the page tables remain the same. Here, the big advantage of Linux is that it does the above very fast.

Other UNIX systems are bloated by SMP locks, so the kernel loses time getting to the task switch point. If that weren't true, the Solaris kernel threads wouldn't be slower than the user-space kernel threads. Of course, the kernel-based threads will scale the load across multiple CPUs, but operating systems like Solaris pay a big fixed cost on systems with few CPUs for the benefit of scaling well with many CPUs. Basically, there is no technical reason why Solaris kernel threads should be lighter than Linux kernel threads. Linux is just doing the minimum possible operations in the context switch path, and it's doing them fast.

Linux kernel threading has constantly improved. Let's look at the different versions again:

In 2.2.x, many places are still single-threaded, but 2.2.x kernels actually scale well only on two-way SMPs. In 2.2.x, the IRQ/timer handling (for example) is completely SMP-threaded, and the IRQ load is distributed across multiple CPUs.

In 2.3.x, most worthwhile code sections within the kernel are being rewritten for SMP threading. For example, all of the VM (virtual memory) is SMP-threaded. The most interesting paths now have a much finer granularity and scale very well.

For the sake of system stability, a kernel has to react well in stress situations. It must, for instance, reduce priorities and resources to processes that misbehave.

How does the scheduler handle a poorly written program looping tightly and forking at each turn of the loop (thereby forking off thousands of processes in a few seconds)? Obviously, the scheduler can't limit the creation of processes time-wise, e.g., a process every 0.5 seconds or similar.

After a fork, however, the “runtime priority” of the process is divided between the parent and the child. This means the parent/child will be penalized compared to the other tasks, and the other tasks will continue to run fine up to the first recalculation of the priorities. This keeps the system from stalling during a fork flooding. The code for this is the concerned code section in linux/kernel/fork.c:

/*
 "share" dynamic priority between parent
 * and child, thus the total amount of dynamic
 * priorities in the system doesn't change, more
 * scheduling fairness. This is only important
 * in the first time slice, in the long run the
 * scheduling behaviour is unchanged.
 */
current->counter >>= 1;
p->counter = current->counter;

Additionally, there is a per-user limit of threads that can be set from init before spawning the first user process. It can be set with  ulimit -u in bash. You can tell it that user moshe can run a maximum ten concurrent tasks (the count includes the shell and every process run by the user).

In Linux, the root user always retains some spare tasks for himself. So, if a user spawns tasks in a loop, the administrator can just log in and use the killall command to remove all tasks of the offending user. Due to the fact that the “runtime priority” of the task is divided between the parent and the child, the kernel reacts smoothly enough to handle this type of situation.

If you wanted to amend the kernel to allow only one fork per processor tick (usually one every 1/100th second; however, this parameter is tunable), called a jiffie, you would have to patch the kernel like this:

--- 2.3.26/kernel/fork.c        Thu Oct 28 22:30:51 1999
+++ /tmp/fork.c Tue Nov  9 01:34:36 1999
@@ -591,6 +591,14 @@
        int retval = -ENOMEM;
        struct task_struct *p;
        DECLARE_MUTEX_LOCKED(sem);
+       static long last_fork;
+
+       while (time_after(last_fork+1, jiffies))
+       {
+               __set_current_state(TASK_INTERRUPTIBLE);
+               schedule_timeout(1);
+       }
+       last_fork = jiffies;
        if (clone_flags & CLONE_PID) {
/* This is only allowed from the boot up thread */

This is the beauty of open source. If you don't like something, just change it!

Here ends the first part of our tour through the Linux kernel. In the next installment, we will take a more detailed look at how the scheduler works. I can promise you some surprising discoveries. Some of these discoveries caused me to revalue completely the probable impact of Linux on the corporate server market. Stay tuned.

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