Threads vs Forks in embedded environment : Some Conclusions

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View
Forks:

Forked processes may not always have its own copy of ALL the segments
of the update engine. Most processes of Linux will do a "copy-on-write"
for a "page", i.e. a process will get its own copy of a page only if it
modifies it. So the RAM requirement is not high. So the only overhead
is creation of the kernel structures.

If we have a MMU, the memory consumption of a process may be lower than
we
think because of the "copy-on-write" semantics.

Switching and interprocess communication time is more. But this is not
an overhead if there is not very frequent switching and communication
between processes, as in our case where each process will execute its
own copy of update engine and work on independent patch parts.

We do not require any additional libraries to supports forks as in case
of threads.
The problem of concurrency and synchronization complexity is not
evident among processes created with fork.

Threads:

Linux has a unique implementation of threads. To the Linux kernel,
there is no concept of a thread. Linux implements all threads as
standard processes. The Linux kernel does not provide any special
scheduling semantics or data structures to represent threads. Instead,
a thread is merely a process that shares certain resources with other
processes. Each thread has a unique task struct and appears to the
kernel as a normal process which just happens to share resources, such
as an address space, with other processes.

Threads are created like normal tasks, with the exception that the
clone () system call is passed flags corresponding to specific
resources to be shared. This leads to a behavior identical to a normal
fork (), except that the address space, file system resources, file
descriptors, and signal handlers are shared. In other words, the new
task and its parent are what are popularly called threads.

This approach to threads contrasts greatly with operating systems such
as Microsoft Windows or Sun Solaris, which have explicit kernel support
for threads (and sometimes call threads lightweight processes).The name
"lightweight process" sums up the difference in philosophies between
Linux and other systems. To these other operating systems, threads are
an abstraction to provide a lighter, quicker execution unit than the
heavy process. To Linux, threads are simply a manner of sharing
resources between processes (which are already quite lightweight)

Threads require support libraries, so extra space is required in flash
memory. If we have to ship just one program that requires the threading
library (as in our case the update engine), then we have to ship the
threading library. Minimizing the threading library cost is only
possible if we can identify all multithreaded programs in the base
Linux distribution. Once we have the library in the flash image to
support just one such program, it costs "nothing" for additional
programs to also link to it. Updation of libraries may also be required
so this may increase the installation time.

Threads have Moderate RAM requirement but it depends upon number of
threads. The advantage of threads is their lower resource consumption.
Multiple threads typically share the state information of a single
process, and share memory & other resources directly. Though threads
share resources, in our case the sharing is not substantial.

Threads take much less CPU time to switch among themselves than between
processes, because there's no need to switch address spaces. In
addition, because they share address space, threads in a process can
communicate more easily with one another. Of course inter thread
communication can be easier than inter process communication, as we can
use shared memory objects, but additional care must be taken to use
thread save functions wherever necessary.

Another problem is concurrency and synchronization complexity.
Sharing, locking, deadlock; race conditions come vividly alive in
threads. Processes don't usually have to deal with this, since most
shared data is passed through pipes. Threads can share file handles,
variables, signals, etc. this may lead to error conditions if not
handled properly.

Applications executed in a thread environment must be thread-safe. This
means that functions (or the methods in object-oriented applications)
must be reentrant-a function with the same input always returns the
same result, even if other threads concurrently execute the same
function. Accordingly, functions must be programmed in such a way that
they can be executed simultaneously by several threads.


Re: Threads vs Forks in embedded environment : Some Conclusions
Plz give ur comments


Re: Threads vs Forks in embedded environment : Some Conclusions
Quoted text here. Click to load it

AFAI understand, that is wrong for Kernel 2.6. See;
http://people.redhat.com/drepper/nptl-design.pdf

With Kernel 2.6. threads are a Kernel concept and they are different
from processes: They have a common PID and the time slice is common for
all threads that belong to a common process.

-Michael

Re: Threads vs Forks in embedded environment : Some Conclusions
No, they don't have a common PID, it's easy to verify.
Quoted text here. Click to load it


Re: Threads vs Forks in embedded environment : Some Conclusions
Quoted text here. Click to load it

I seemed to remember to have read this somewhere but maybe I'm wrong.

But nonetheless the threads of an application are part of a common
concept in the Kernel and not completely independent processes. So they
are (more) compliant to POSIX and feature less overhead. So IMHO this is
what the OP should use instead of independent processes or of library
based user-space threads. Of course he would need to use Kernel 2.6 to
make it work.

Re: Threads vs Forks in embedded environment : Some Conclusions
Quoted text here. Click to load it

Why do you think so ? There are libraries that do complete user land
implementation of threads (i.e. a multitasking scheduler done for a
single Linux process). This is much more Posix conform that the "Linux
Threads" implementation for Kernel 2.4 (each thread a Linux process).
This "Sub-OS" of course needs a lot of code memory. But with Kernel 2.6
NPTL feature this is not necessary any more, as the Kernel supplies a
Posix conform thread model.

-Michael

Site Timeline