Andrey Semashev wrote:
Hello Tobias,
Sunday, August 26, 2007, 1:37:17 PM, you wrote:
Hmm, I'm not sure of the purpose of this project. Is it supposed to pass several tools under its umbrella to boost via fast-track review?
Sort of. It's just an idea, so far.
Its purpose is to avoid lots of fast-track reviews (and reviewing overhead) for utility components by grouping them into a "pseudo library", thus encouraging developers to brush up / factor out useful stuff.
In this particular case the tool will be reviewed during the Boost.FSM review (if it will, since it's not for public use anyway), so including it in X-Files won't reduce the amount of reviews. On the other hand, if Boost.FSM is rejected but there is interest to this tool, I would gladly extract it to the X-Files project.
If your library gets accepted, LWCO is accepted as an implementation detail. AFAIK you need at least a fast-track review to make it a public thing (not sure that's what you want, though). However, I'd at least very much welcome a test suite for LWCO.
Yes, but consider that this code will be executed only once. The rest of the execution time this mutex is useless.
Consider the deadlock if 'once' is used recursively to initialize different resources...
The mutex is recursive.
Sorry, missed it.
Further, it's quite unintuitive that a trivial initialization might get slowed down by one in another thread that takes a lot of time.
You can call 'pthread_mutex_destroy' once you're done with the mutex to free up eventually acquired system resources.
I'll think of it. The first thing that comes to mind is that I'd have to count threads that are hanging locked in the mutex since destroying it right away would leave those threads in undefined behavior.
It might be possible to use the flag for the counter...
Also, some platforms will not call 'mutex_destroyer' within a dynamic library (you probably know)... No, I'm not aware of this. Could you elaborate, please? Which platforms are those?
No ctors/dtors are run in static context for shared libraries on most UNIX platforms.
That's quite a surprise for me. I didn't encounter such behavior on Linux (Red Hat). Do you have any workaround for this? I'm thinking of GCC-specific attributes for this purpose, but that's one step away from portability.
AFAIK there's not much one can do about it except for adding a function for explicit disposal.
The fundamental problem arises here - I need to safely create a synchronization object. Non-POSIX APIs don't provide things like PTHREAD_MUTEX_INITIALIZER or I didn't find them in the docs.
I see. Would it be an option to use 'yield' instead of 'sleep'?
The "yield" function is not guaranteed to switch execution context, it may return immediately. If a lower-priority thread entered once functor, you may spin for a relatively long time in a "yield" loop instead of just letting the lower-priority thread finish its job.
I figured something like that. Does 'Sleep' guarantee preemption - or does it depend on the argument and the resolution of the system timer?
For some platforms (such as x86) memory access is atomic, so atomic operations are just a waste of time for simple read/write operations as the 'is_init' and 'set_called' stuff. The point is not only in atomic reads and writes, but in performing memory barriers too. Otherwise the result of executing the once functor could not have been seen by other CPUs.
Then the memory barriers will suffice for x86, correct? As this code is executed on every call, any superfluous bus-locking should be avoided.
Actually, I got the impression that barriers themselves do a major deal of performance impact. Besides, not all compilers support barrier intrinsics.
Alternatively, doing an "uncertain read" to check whether we might need initialization before setting up the read barrier might be close enough to optimal.
Bad wording on my side. Substitute "Alternatively" with "Additionally".
Well, that's a tricky point. I'm not an expert in threading issues, but it's not obvious to me whether a memory barrier should act regardless of its scope. For example:
void foo(int& x, int& y) { if (x == 0) { read_memory_barrier(); y = 10; x = 1; write_memory_barrier(); }
// use y }
Now, is it guaranteed that those barriers are in effect regardless of x value? I think not. Either the compiler may reorder statements in such way that y is used before the "if" statement, or the same thing may be done by CPU since the barrier instructions may not be executed.
That's not quite what I meant: // 'initialized' started being false if (initialized) { // 'initialized' is true for sure } else { // we can't know 'initialized' is still false, so let's // synchronize and check again read_memory_barrier(); if (initialized) { // 'initialized' is true } else { // 'initialized' is false } } Now we only have to cross the bsrrier during (and immediately after) initialization.
Well, you may be right here. I could try to reduce memory allocations in error handling. But the only possible problem I see there is memory depletion. In such case you'll get std::bad_alloc which adheres the declared interface of the implementation. So, strictly speaking, if you have enough memory you get a detailed error description. If not, you get bad_alloc.
Depending on 'lexical_cast', 'iostream' and 'string' still slightly bugs me, though.
Ok, I'll change the code that formats the error string not to use lexical_cast. But it will still depend on std::string since it's in the exception class.
Getting rid of 'lexical_cast' and 'iostream' seems good savings, already...
Another potential issue: It seems Win32 and MacOS variants are currently not exception-safe. That is, the initialization routine isn't rerun if it has thrown the first time 'once' was called.
Yep, thanks for spotting that. I'll fix that in a couple of days and update the library archive in the Vault. I'll post here a notification when it's done.
Looking forward to it! Regards, Tobias