Is it safe to use the ARPACK eigensolver from different threads at the same time from a program written in C? Or, if ARPACK itself is not thread-safe, is there an API-compatible thread-safe implementation out there? A quick Google search didn't turn up anything useful, but given the fact that ARPACK is used heavily in large scientific calculations, I'd find it highly surprising to be the first one who needs a thread-safe sparse eigensolver.
I'm not too familiar with Fortran, so I translated the ARPACK source code to C using f2c, and it seems that there are quite a few static variables. Basically, all the local variables in the translated routines seem to be static, implying that the library itself is not thread-safe.
Fortran 77 does not support recursion, and hence a standard conforming compiler can allocate all variables in the data section of the program; in principle, neither a stack nor a heap is needed [1].
It might be that this is what f2c is doing, and if so, it might be that it's the f2c step that makes the program non thread-safe, rather than the program itself. Of course, as others have mentioned, check out for COMMON blocks as well. EDIT: Also, check for explicit SAVE directives. SAVE means that the value of the variable should be retained between subsequent invocations of the procedure, similar to static in C. Now, allocating all procedure local data in the data section makes all variables implicitly SAVE, and unfortunately, there is a lot of old code that assumes this even though it's not guaranteed by the Fortran standard. Such code, obviously, is not thread-safe. Wrt. ARPACK specifically, I can't promise anything but ARPACK is generally well regarded and widely used so I'd be surprised if it suffered from these kinds of dusty-deck problems.
Most modern Fortran compilers do use stack allocation. You might have better luck compiling ARPACK with, say, gfortran and the -frecursive option.
EDIT:
[1] Not because it's more efficient, but because Fortran was originally designed before stacks and heaps were invented, and for some reason the standards committee wanted to retain the option to implement Fortran on hardware with neither stack nor heap support all the way up to Fortran 90. Actually, I'd guess that stacks are more efficient on todays heavily cache-dependent hardware rather than accessing procedure local data that is spread all over the data section.
I have converted ARPACK to C using f2c. Whenever you use f2c and you care about thread-safety you must use the -a switch. This makes local variables have automatic storage, i.e. be stack based locals rather than statics which is the default.
Even so, ARPACK itself is decidedly not threadsafe. It uses a lot of common blocks (i.e. global variables) to preserve state between different calls to its functions. If memory serves, it uses a reverse communication interface which tends to lead developers to using global variables. And of course ARPACK probably was written long before multi-threading was common.
I ended up re-working the converted C code to systematically remove all the global variables. I created a handful of C structs and gradually moved the global variables into these structs. Finally I passed pointers to these structs to each function that needed access to those variables. Although I could just have converted each global into a parameter wherever it was needed it was much cleaner to keep them all together, contained in structs.
Essentially the idea is to convert global variables into local variables.
ARPACK uses BLAC right? Then those libraries need to be thread safe too.
I believe your idea to check with f2c might not be a bullet proof way of telling if the Fortran code is thread safe, I would guess it also depends on the Fortran compiler and libraries.
I don't know what strategy f2c uses in translating Fortran. Since ARPACK is written in FORTRAN 77, the first thing to do is check for the presence of COMMON blocks. These are global variables, and if used, the code is most likely not thread safe. The ARPACK webpage, http://www.caam.rice.edu/software/ARPACK/, says that there is a parallel version -- it seems likely that that version is threadsafe.
Related
I am wondering what methods there are to add typing information to generated C methods. I'm transpiling a higher-level programming language to C and I'd like to add a moving garbage collector. However to do that I need the method variables to have typing information, otherwise I could modify a primitive value that looks like a pointer.
An obvious approach would be to encapsulate all (primitive and non-primitive) variables in a struct that has an extra (enum) variable for typing information, however this would cause memory and performance overhead, the transpiled code is namely meant for embedded platforms. If I were to accept the memory overhead the obvious option would be to use a heap handle for all objects and then I'd be able to freely move heap blocks. However I'm wondering if there's a more efficient better approach.
I've come up with a potential solution, namely to predeclare and group variables based whether they're primitives or not (I can do that in the transpiler), and add an offset variable to each method at the end (I need to be able to find it accurately when scanning the stack area), that tells me where the non-primitive variables begin and where they end, so I can only scan those. This means that each method will use an additional 16/32-bit (depending on arch) of memory, however this should still be more memory efficient than the heap handle approach.
Example:
void my_func() {
int i = 5;
int z = 3;
bool b = false;
void* person;
void* person_info = ...;
.... // logic
volatile int offset = 0x034;
}
My aim is for something that works universally across GCC compilers, thus my concerns are:
Can the compiler reorder the variables from how they're declared in
the source code?
Can I force the compiler to put some data in the
method's stack frame (using volatile)?
Can I find the offset accurately when scanning the stack?
I'd like to avoid assembly so this approach can work (by default) across multiple platforms, however I'm open for methods even if they involve assembly (if they're reliable).
Typing information could be somehow encoded in the C function name; this is done by C++ and other implementations and called name mangling.
Actually, you could decide, since all your C code is generated, to adopt a different convention: generate long C identifiers which are practically unique and sort-of random program-wide, such as tiziw_7oa7eIzzcxv03TmmZ and keep their typing information elsewhere (e.g. some database). On Linux, such an approach is friendly to both libbacktrace and dlsym(3) + dladdr(3) (and of course nm(1) or readelf(1) or gdb(1)), so used in both bismon and RefPerSys projects.
Typing information is practically tied to calling conventions and ABIs. For example, the x86-64 ABI for Linux mandates different processor registers for passing floating points or pointers.
Read the Garbage Collection handbook or at least P.Wilson Uniprocessor Garbage Collection Techniques survey. You could decide to use tagged integers instead of boxing them, and you could decide to have a conservative GC (e.g. Boehm's GC) instead of a precise one. In my old GCC MELT project I generated C or C++ code for a generational copying GC. Similar techniques are used both in Bismon and in RefPerSys.
Since you are transpiling to C, consider also alternatives, such as libgccjit or LLVM. Look into libjit and asmjit.
Study also the implementation of other transpilers (compilers to C), including Chicken/Scheme and Bigloo.
Can the GCC compiler reorder the variables from how they're declared in the source code?
Of course yes, depending upon the optimizations you are asking. Some variables won't even exist in the binary (e.g. those staying in registers).
Can I force the compiler to put some data in the method's stack frame (using volatile)?
Better generate a single struct variable containing all your language variables, and leave optimizations to the compiler. You will be surprised (see this draft report).
Can I find the offset accurately when scanning the stack?
This is the most difficult, and depends a lot of compiler optimizations (e.g. if you run gcc with -O1 or -O3 on the generated C code; in some cases a recent GCC -e.g GCC 9 or GCC 10 on x86-64 for Linux- is capable of tail-call optimizations; check by compiling using gcc -O3 -S -fverbose-asm then looking into the produced assembler code). If you accept some small target processor and compiler specific tricks, this is doable. Study the implementation of the Ocaml compiler.
Send me (to basile#starynkevitch.net) an email for discussion. Please mention the URL of your question in it.
If you want to have an efficient generational copying GC with multi-threading, things become extremely tricky. The question is then how many years of development can you afford spending.
If you have exceptions in your language, take also a great care. You could with great caution generate calls to longjmp.
See of course this answer of mine.
With transpiling techniques, the evil is in the details
On Linux (specifically!) see also my manydl.c program. It demonstrates that on a Linux x86-64 laptop you could generate, in practice, hundred of thousands of dlopen(3)-ed plugins. Read then How to write shared libraries
Study also the implementation of SBCL and of GNU Prolog, at least for inspiration.
PS. The dream of a totally architecture-neutral and operating-system independent transpiler is an illusion.
Imagine a situation where you can't or don't want to use any of the libraries provided by the compiler as "standard", nor any external library. You can't use even the compiler extensions (such as gcc extensions).
What is the remaining part you get if you strip C language of all the things a lot of people use as a matter of course?
In such a way, probably a list of every callable function supported by any big C compiler (not only ANSI C) out-of-box would be satisfying as as answer as it'd at least approximately show the use-case of the language.
First I thought about sizeof() and printf() (those were already clarified in the comments - operator + stdio), so... what remains? In-line assembly seem like an extension too, so that pretty much strips even the option to use assembly with C if I'm right.
Probably in the matter of code it'd be easier to understand. Imagine a code compiled with only e.g. gcc main.c (output flag permitted) that has no #include, nor extern.
int main() {
// replace_me
return 0;
}
What can I call to actually do something else than "boring" type math and casting from type to type?
Note that switch, goto, if, loops and other constructs that do nothing and only allow repeating a piece of code aren't the thing I'm looking for (if it isn't obvious).
(Hopefully the edit clarified wtf I'm actually asking, but Matteo's answer pretty much did it.)
If you remove all libraries essentially you have something similar to a freestanding implementation of C (which still has to provide some libraries - say, string.h, but that's nothing you couldn't easily implement yourself in portable C), and that's what normally you start with when programming microcontrollers and other computers that don't have a ready-made operating system - and what operating system writers in general use when they compile their operating systems.
There you typically have two ways of doing stuff besides "raw" computation:
assembly blocks (where you can do literally anything the underlying machine can do);
memory mapped IO (you set a volatile pointer to some hardware dependent location and read/write from it; that affects hardware stuff).
That's really all you need to build anything - and after all, it all boils down to that stuff anyway, the C library of a regular hosted implementation is normally written in C itself, with some assembly used either for speed or to communicate with the operating system1 (typically the syscalls are invoked through some kind of interrupt).
Again, it's nothing you couldn't implement yourself. But the point of having a standard library is both to avoid to continuously reinvent the wheel, and to have a set of portable functions that spare you to have to rewrite everything knowing the details of each target platform.
And mainstream operating systems, in turn, are generally written in a mix or C and assembly as well.
C has no "built-in" functions as such. A compiler implementation may include "intrinsic" functions that are implemented directly by the compiler without provision of an external library, although a prototype declaration is still required for intrinsics, so you would still normally include a header file for such declarations.
C is a systems-level language with a minimal run-time and start-up requirement. Because it can directly access memory and memory mapped I/O there is very little that it cannot do (and what it cannot do is what you use assembly, in-line assembly or intrinsics for). For example, much of the library code you are wondering what you can do without is written in C. When running in an OS environment however (using C as an application-level rather then system-level language), you cannot practically use C in that manner - the OS has control over such things as I/O and memory-management and in modern systems will normally prevent unmediated access to such resources. Of course that OS itself is likely to largely written in C (and/or C++).
In a standalone of bare-metal environment with no OS, C is often used very early in the bootstrap process initialising hardware and establishing an application execution environment. In fact on ARM Cortex-M processors it is possible to boot directly into C code from reset, since the hardware loads an initial stack-pointer and start address from the vector table on start-up; this being enough to run C code that does not rely on library or static data initialisation - such initialisation can however be written in C before calling main().
Note that sizeof is not a function, it is an operator.
I don't think you really understand the situation.
You don't need a header to call a function in C. You can call with unchecked parameters - a bad idea and an obsolete feature, but still supported. And if a compiler links a library by default instead of only when you explicitly tell it to, that's only a little switch within the compiler to "link libc". Notoriously Unix compilers need to be told to link the math library, it wasn't linked by default because some very early programs didn't use floating point.
To be fair, some standard library functions like memcpy tend to be special-cased these days as they lend themselves to inlining and optimisation.
The standard library is documented and is usually available, though in effect deprecated by Microsoft for security reasons. You can write pretty much any function quite easily with only stdlib functions, what you can't do is fancy IO.
I'm new to Linux kernel development.
One thing that bothers me is a way a variables are declared and initialized.
I'm under impression that code uses variable declaration placement rules for C89/ANSI C (variables are declared at the beginning of block), while C99 relaxes the rule.
My background is C++ and there many advises from "very clever people" to declare variable as locally as possible - better declare and initialize in the same instruction:
Google C++ Style Guide
C++ Coding Standards: 101 Rules, Guidelines, and Best Practices - item 18
A good discussion about it here.
What is the accepted way to initialize variables in Linux kernel?
I couldn't find a relevant passage in the Linux kernel coding style. So, follow the convention used in existing code -- declare variables at beginning of block -- or run the risk of your code seeming out-of-place.
Reasons why variables at beginning of block is a Good Thing:
the target architecture may not have a C99 compiler
... can't think of more reasons
You should always try to declare variables as locally as possible. If you're using C++ or C99, that would usually be right before the first use.
In older C, doing that doesn't fall under "possible", and there the place to declare those variables would usually be the beginning of the current block.
(I say 'usually' because of some cases with functions and loops where it's better to make them a bit more global...)
In most normal cases, declare them in the beginning of the function where you are using them. There are exceptions, but they are rare.
if your function is short enough, the deceleration is far away from the first use anyway. If your function is longer then that - it's a good sign your function is too long.
The reason many C++ based coding standards recommend declaring close to use is that C++ data types can be much "fatter" (e.g. thing of class with multiple inheritances etc.) and so take up a lot more space. If you define such an instance at the beginning of a function but use it only much later (and maybe not at all) you are wasting a lot of RAM. This tends to be much less of an issue in C with it's native type only data types.
There is an oblique reference in the Coding Style document. It says:
Another measure of the function is the number of local variables. They
shouldn't exceed 5-10, or you're doing something wrong. Re-think the
function, and split it into smaller pieces. A human brain can
generally easily keep track of about 7 different things, anything more
and it gets confused. You know you're brilliant, but maybe you'd like
to understand what you did 2 weeks from now.
So while C99 style in-place initialisers are handy in certain cases the first thing you should probably be asking yourself is why it's hard to have them all at the top of the function. This doesn't prevent you from declaring stuff inside block markers, for example for in-loop calculations.
In older C it is possible to declare them locally by creating a block inside the function. Blocks can be added even without ifs/for/while:
int foo(void)
{
int a;
int b;
....
a = 5 + b;
{
int c;
....
}
}
Although it doesn't look very neat, it still is possible, even in older C.
I can't speak to why they have done things one way in the Linux kernel, but in the systems we develop, we tend to not use C99-specific features in the core code. Individual applications tend to have stuff written for C99, because they will typically be deployed to one known platform, and the gcc C99 implementation is known good.
But the core code has to be deployable on whatever platform the customer demands (within reason). We have supplied systems on AIX, Solaris, Informix, Linux, Tru-64, OpenVMS(!) and the presence of C99 compliant compilers isn't always guaranteed.
The Linux kernel needs to be substantially more portable again - and particularly down to small footprint embedded systems. I guess the feature just isn't important enough to override these sorts of considerations.
I would like to define a few variables as thread-specific using the __thread storage class. But three questions make me hesitate:
Is it really standard in c99? Or more to the point, how good is the compiler support?
Will the variables be initialised in every thread?
Do non-multi threaded programs treat them as plain-old-globals?
To answer your specific questions:
No, it is not part of C99. You will not find it mentioned anywhere in the n1256.pdf (C99+TC1/2/3) or the original C99 standard.
Yes, __thread variables start out with their initialized value in every new thread.
From a standpoint of program behavior, thread-local storage class variables behave pretty much the same as plain globals in non-multi-threaded programs. However, they do incur a bit more runtime cost (memory and startup time), and there can be issues with limits on the size and number of thread-local variables. All this is rather complicated and varies depending on whether your program is static- or dynamic-linked and whether the variables reside in the main program or a shared library...
Outside of implementing C/POSIX (e.g. errno, etc.), thread-local storage class is actually not very useful, in my opinion. It's pretty much a crutch for avoiding cleanly passing around the necessary state in the form of a context pointer or similar. You might think it could be useful for getting around broken interfaces like qsort that don't take a context pointer, but unfortunately there is no guarantee that qsort will call the comparison function in the same thread that called qsort. It might break the job down and run it in multiple threads. Same goes for most other interfaces where this sort of workaround would be possible.
You probably want to read this:
http://www.akkadia.org/drepper/tls.pdf
1) MSVC doesn't support C99. GCC does and other compilers attempt GCC compatibility.
edit A breakdown of compiler support for __thread is available here:
http://chtekk.longitekk.com/index.php?/archives/2011/02/C8.html
2) Only C++ supports an initializer and it must be constant.
3) Non-multi-threaded applications are single-threaded applications.
im working on a c lib which would be nice to also work on embedded systems
but im not very deep into embedded development so my question
are most embedded compilers able to cope with local static variables - which i would then just assume in further development
OR
is there a #define which i can use for a #ifdef to create a global variable in case of
thx
They should, as local static variables are part of the C standard.
Of course, there is nothing preventing them from creating a C-like language that does not have all the features. But since that would be non-standard, then the way to identify that a feature is lacking would be non-standard as well.
Since static variables are part of the standard, you should be safe.
The problem with support is probably not to be found with your compiler (most of which handle the standard pretty well), but with whatever code you have to set up your runtime environment. Make sure that when you're loading the code that you properly unpack the executable, read-only data, read-write data, and zero-init sections of the executable before jumping into the C code.
Local static variables are part of th C standard, so yes.
\pedantic{
If your code is well organized, with separate files (compilation units) for different subsystems, you might do better to have a static variable with file scope. This will make it easier to factor the code that uses it into separate functions. If the code that uses the variable is complicated, this will permit you to split it into smaller static functions, which are easier to read, understand and debug.
}
Yes. local statics are really not much different than globals once the compiler is done chewing on your source code. I could think up exotic processors where globals would be an issue, but I doubt you will encounter many.
The truly interesting thing about globals on embedded processors is that you often have the option of having the compiler allocate them in ROM, EEPROMs, etc.