The local NT-head ( and decision maker ) here in the office is basing his
opinion of linux off a sidebar in Windows NT magazine. I think that his
information is biased and misleading, if not wrong, but I really don't know
enough about the Linux kernel to argue against it. I have included parts of
the article below. If someone more knowledgable than I could take a look and
tell me whether it is correct or not I really would appreciate it!
-Start article-
Linux is an active and evolving UNIX implementation that is gaining wide
acceptance. However, Linux currently has several major limitations that
prevent it from being a contender in any arena other than the small-server
segment. Whereas Windows NT and all leading commercial operating systems
(OSs) implement kernel-mode threads, Linux does not. The Linux scheduler
does not divide CPU time among threads but among processes. Each process in
Linux has an implied execution state that, on other UNIX variants, is the
equivalent of a thread. Thus, a Linux process is similar to a process on
other UNIX versions that has exactly one thread and cannot create more.
Linu'x ommision of kernel-mode threads seriously affaces application
developers' ability to write software that takes maximum advantage of a CPU.
Linux application developers must use user-mode threads that the system
implements in user space. User-mode threads are also called cooperatively
scheduled threads, because kernel schedulers do not divide CPU time among
the threads. Instead, applications map multiple user-mode threads onto a
common kernel-mode thread. Then, an application can cause the execution
focus of the common kernel-mode thread to move around the application's
code. The drawback of this method is that if a user-mode thread calls a
kernel function that then blocks the thread ( e.g. by waiting of I/O input
), then the application cannot get any other work done. In implementations
based on kernel-mode threads, blocking one thread of an application does not
stop other threads that the application created from doing useful work.
Another limitation of the Linux kernel is that the scheduler cannot
preemt the kernel, as can the scheduler in most modern UNIX variants and NT.
Because Linux's kernel is not preemtible, it is not as responsive to
high-priority processes as other OS kernels are. When a high-priority
process in Linux is ready to execute - for example, after it finishes an I/O
operation on which it was blocked - the process my wait until the currently
executing process reaches a well-defined preemption point in the kernel ( if
that process is executing on the kernel ). On other UNIX systems, the
high-priority process ( or thread, on other OSs ) would preempt the
executing thread almost immediately.
Finally, the Linux kernel is not reeentrant, which means that only
one processor in a multi-processor Linux system can execute kernel code at a
time. This limitation is perhaps the biggest impediment to Linux's
scalability on multiprocessors, because Linux serializes all its kernel
services on a multiprocessor, making them run as if they were on a
uniprocessor. Thus, I/O-intensive applications or applications that make
heavy use of kernel services will not gain a significant performance benefit
from executing on a multiprocessor. Linux is not reentrant because Linux's
creator originally wrote Linux to run on uniprocessors. Making a kernel run
efficiently on a multiprocessor means implementing relatively complex data
structure-locking hierarchies, a task that is time consuming on a structure
that is difficult to retrofit.
How do these various limitations affect Linux's chance to compete
with NT in the enterprise? With adequate OS support, enterprise server
applications- including Web servers, database servers, and email servers-
can effectively use multiprocessors. For the next couple of years, Lunix is
stuck with being a valid choice for only small uniprocessor servers. None
but the most CPU-intensive Linux applications will scale past one CPU, which
makes Linux unacceptable as a multiprocessor OS, and therefor as an
enterprise-class OS.
----end article---
Painstakingly typed by me ( Timothy MacDonald ) from the December 1998 issue
of WindowsNT magazine ( page 122 ). Please excuse any typos.
Thanks for your time,
Timothy MacDonald
[EMAIL PROTECTED]
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