[cgl_discussion] [Fwd: [ANNOUNCE] Native POSIX Thread Library 0.1]
Khalid Aziz
khalid at fc.hp.com
Fri Sep 20 09:25:21 PDT 2002
Here is another implementation of POSIX threads. Might be interesting
for POSIX threads project to look at. People have already started asking
for comparison with NGPT on lkml.
--
Khalid
Ulrich Drepper wrote:
>
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> We are pleased to announce the first publically available source
> release of a new POSIX thread library for Linux. As part of the
> continuous effort to improve Linux's capabilities as a client, server,
> and computing platform Red Hat sponsored the development of this
> completely new implementation of a POSIX thread library, called Native
> POSIX Thread Library, NPTL.
>
> Unless major flaws in the design are found this code is intended to
> become the standard POSIX thread library on Linux system and it will
> be included in the GNU C library distribution.
>
> The work visible here is the result of close collaboration of kernel
> and runtime developers. The collaboration proceeded by developing the
> kernel changes while writing the appropriate parts of the thread
> library. Whenever something couldn't be implemented optimally some
> interface was changed to eliminate the issue. The result is this
> thread library which is, unlike previous attempts, a very thin layer
> on top of the kernel. This helps to achieve a maximum of performance
> for a minimal price.
>
> A white paper (still in its draft stage, though) describing the design
> is available at
>
> http://people.redhat.com/drepper/nptl-design.pdf
>
> It provides a larger number of details on the design and insight into
> the design process. At this point we want to repeat only a few
> important points:
>
> - - the new library is based on an 1-on-1 model. Earlier design
> documents stated that an M-on-N implementation was necessary to
> support a scalable thread library. This was especially true for
> the IA-32 and x86-64 platforms since the ABI with respect to threads
> forces the use of segment registers and the only way to use those
> registers was with the Local Descriptor Table (LDT) data structure
> of the processor.
>
> The kernel limitations the earlier designs were based on have been
> eliminated as part of this project, opening the road to a 1-on-1
> implementation which has many advantages such as
>
> + less complex implementation;
> + avoidance of two-level scheduling, enabling the kernel to make all
> scheduling decisions;
> + direct interaction between kernel and user-level code (e.g., when
> delivering signals);
> + and more and more.
>
> It is not generally accepted that a 1-on-1 model is superior but our
> tests showed the viability of this approach and by comparing it with
> the overhead added by existing M-on-N implementations we became
> convinced that 1-on-1 is the right approach.
>
> Initial confirmations were test runs with huge numbers of threads.
> Even on IA-32 with its limited address space and memory handling
> running 100,000 concurrent threads was no problem at all, creating
> and destroying the threads did not take more than two seconds. This
> all was made possible by the kernel work performed as part of this
> project.
>
> The only limiting factors on the number of threads today are
> resource availability (RAM and processor resources) and architecture
> limitations. Since every thread needs at least a stack and data
> structures describing the thread the number is capped. On 64-bit
> machines the architecture does not add any limitations anymore (at
> least for the moment) and with enough resources the number of
> threads can be grown arbitrarily.
>
> This does not mean that using hundreds of thousands of threads is a
> desirable design for the majority of applications. At least not
> unless the number of processors matches the number of threads. But
> it is important to note that the design on the library does not have
> a fixed limit.
>
> The kernel work to optimize for a high thread count is still
> ongoing. Some places in which the kernel iterates over process and
> threads remain and other places need to be cleaned up. But it has
> already been shown that given sufficient resources and a reasonable
> architecture an order of magnitude more threads can be created than
> in our tests on IA-32.
>
> - - The futex system call is used extensively in all synchronization
> primitives and other places which need some kind of
> synchronization. The futex mechanism is generic enough to support
> the standard POSIX synchronization mechanisms with very little
> effort.
>
> The fact that this is possible is also essential for the selection
> of the 1-on-1 model since only with the kernel seeing all the
> waiters and knowing that they are blocked for synchronization
> purposes will allow the scheduler to make decisions as good as a
> thread library would be able to in an M-on-N model implementation.
>
> Futexes also allow the implementation of inter-process
> synchronization primitives, a sorely missed feature in the old
> LinuxThreads implementation (Hi jbj!).
>
> - - Substantial effort went into making the thread creation and
> destruction as fast as possible. Extensions to the clone(2) system
> call were introduced to eliminate the need for a helper thread in
> either creation or destruction. The exit process in the kernel was
> optimized (previously not a high priority). The library itself
> optimizes the memory allocation so that in many cases the creation
> of a new thread can be achieved with one single system call.
>
> On an old IA-32 dual 450MHz PII Xeon system 100,000 threads can be
> created and destroyed in 2.3 secs (with up to 50 threads running at
> any one time).
>
> - - Programs indirectly linked against the thread library had problems
> with the old implementation because of the way symbols are looked
> up. This should not be a problem anymore.
>
> The thread library is designed to be binary compatible with the old
> LinuxThreads implementation. This compatibility obviously has some
> limitations. In places where the LinuxThreads implementation diverged
> from the POSIX standard incompatibilities exist. Users of the old
> library have been warned from day one that this day will come and code
> which added work-arounds for the POSIX non-compliance better be
> prepared to remove that code. The visible changes of the library
> include:
>
> - - The signal handling changes from per-thread signal handling to the
> POSIX process signal handling. This change will require changes in
> programs which exploit the non-conformance of the old implementation.
>
> One consequence of this is that SIGSTOP works on the process. Job
> control
> in the shell and stopping the whole process in a debugger work now.
>
> - - getpid() now returns the same value in all threads
>
> - - the exec functions are implemented correctly: the exec'ed process gets
> the PID of the process. The parent of the multi-threaded application
> is only notified when the exec'ed process terminates.
>
> - - thread handlers registered with pthread_atfork are not anymore run
> if vfork is used. This isn't required by the standard (which does
> not define vfork) and all which is allowed in the child is calling
> exit() or an exec function. A user of vfork better knows what s/he
> does.
>
> - - libpthread should now be much more resistant to linking problems: even
> if the application doesn't list libpthread as a direct dependency
> functions which are extended by libpthread should work correctly.
>
> - - no manager thread
>
> - - inter-process mutex, read-write lock, conditional variable, and barrier
> implementations are available
>
> - - the pthread_kill_other_threads_np function is not available. It was
> needed to work around the broken signal handling. If somebody shows
> some existing code which makes legitimate use of this function we
> might add it back.
>
> - - requires a kernel with the threading capabilities of Linux 2.5.36.
>
> The sources for the new library are for the time being available at
>
> ftp://people.redhat.com/drepper/nptl/
>
> The current sources contain support only for IA-32 but this will
> change very quickly. The thread library is built as part of glibc so
> the complete set of glibc sources is available as well. The current
> snapshot for glibc 2.3 (or glibc 2.3 when released) is necessary. You
> can find it at
>
> ftp://sources.redhat.com/pub/glibc/snapshots
>
> Final releases will be available on ftp.gnu.org and its mirrors.
>
> Building glibc with the new thread library is demanding on the
> compilation environment.
>
> - - The 2.5.36 kernel or above must be installed and used. To compile
> glibc it is necessary to create the symbolic link
>
> /lib/modules/$(uname -r)/build
>
> to point to the build directory.
>
> - - The general compiler requirement for glibc is at least gcc 3.2. For
> the new thread code it is even necessary to have working support for
> the __thread keyword.
>
> Similarly, binutils with functioning TLS support are needed.
>
> The (Null) beta release of the upcoming Red Hat Linux product is
> known to have the necessary tools available after updating from the
> latest binaries on the FTP site. This is no ploy to force everybody
> to use Red Hat Linux, it's just the only environment known to date
> which works. If alternatives are known they can be announced on the
> mailing list.
>
> - - To configure glibc it is necessary to run in the build directory
> (which always should be separate from the source directory):
>
> /path/to/glibc/configure --prefix=/usr --enable-add-ons=linuxthreads2 \
> --enable-kernel=current --with-tls
>
> The --enable-kernel parameter requires that the 2.5.36+ kernel is
> running. It is not strictly necessary but helps to avoid mistakes.
> It might also be a good idea to add --disable-profile, just to speed
> up the compilation.
>
> When configured as above the library must not be installed since it
> would overwrite the system's library. If you want to install the
> resulting library choose a different --prefix parameter value.
> Otherwise the new code can be used without installation. Running
> existing binaries is possible with
>
> elf/ld.so --library-path .:linuxthreads2:dlfcn:math <binary> <args>...
>
> Alternatively the binary could be build to find the dynamic linker
> and DSO by itself. This is a much easier way to debug the code
> since gdb can start the binary. Compiling is a bit more complicated
> in this case:
>
> gcc -nostdlib -nostartfiles -o <OUTPUT> csu/crt1.o csu/crti.o \
> $(gcc --print-file-name=crtbegin.o) <INPUTS> \
> -Wl,-rpath,$PWD,-dynamic-linker,$PWD/ld-linux.so.2 \
> linuxthreads2/libpthread.so.0 ./libc.so.6 ./libc_nonshared.a \
> elf/ld-linux.so.2 $(gcc --print-file-name=crtend.o) csu/crtn.o
>
> This command assumes that it is run in the build directory. Correct
> the paths if necessary. The compilation will use the system's
> headers which is a good test but might lead to strange effects if
> there are compatibility bugs left.
>
> Once all these prerequisites are met compiling glibc should be easy.
> But there are some tests which will flunk. For good reasons we aren't
> officially releasing the code yet. The bugs are either in the TLS
> code which is not enabled in the standard glibc build, or obviously in
> the thread library itself. To run the tests for the thread library
> run
>
> make subdirs=linuxthreads2 check
>
> One word on the name 'linuxthreads2' of the directory. This is only a
> convenience thing so that the glibc configure scripts don't complain
> about missing thread support. It will we changed to reflect the real
> name of the library ASAP.
>
> What can you expect?
>
> This is a very early version of the code so the obvious answer is:
> some problems. The test suite for the new thread code should pass but
> beside that and some performance measurement tool we haven't run much
> code. Ideally we would get people to write many more of these small
> test programs which are included in the sources. Compiling big
> programs would mean not being able to locate problems easy. But I
> certainly won't object to people running and debugging bigger
> applications. Please report successes and failures to the mailing
> list.
>
> People who are interested in contributing must be aware that for any
> non-trivial change we need an assignment of the code to the FSF. The
> process is unfortunately necessary in today's world.
>
> People who are contaminated by having worked on proprietary thread
> library implementation should not participate in discussions on the
> mailing list unless they willfully disclose the information. Every
> bit of information is publically available from the mailing list
> archive.
>
> Which brings us to the final point: the mailing list for *all*
> discussions related to this thread library implementation is
>
> phil-list at redhat.com
>
> Go to
>
> https://listman.redhat.com/mailman/listinfo/phil-list
>
> to subscribe, unsubscribe, or review the archive.
>
> - --
> - ---------------. ,-. 1325 Chesapeake Terrace
> Ulrich Drepper \ ,-------------------' \ Sunnyvale, CA 94089 USA
> Red Hat `--' drepper at redhat.com `------------------------
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====================================================================
Khalid Aziz Linux Systems Division
(970)898-9214 Hewlett-Packard
khalid at fc.hp.com Fort Collins, CO
"The Linux kernel is subject to relentless development"
- Alessandro Rubini
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