CreateFiber() : number of fibers limited

[Danny Smith]

--- Dan Plimak <danp@systematik.co.nz> wrote: > On Sun, Jun 03, 2001 at
12:16:07PM +0200, Holger Vogt wrote:
> > Hi,
> > 
> > independently of the requested stack size (first parameter in
> > CreateFiber()), CYGWIN will give only 53 fibers. Only when I increase
> > the stack size above 9.5 MByte, I will get a further reduction in the
> > number of fibers created.
> 
> I was intrigued by this difference between gcc and Borland C++Builder and MS
> Visual C++, so I tried the following code under all three compilers on
> Win2K:
> 
> ---
> 
> #define _WIN32_WINNT 0x0500 /* Coax winbase.h into including
>                                ConvertThreadToFiber() */
> #include <windows.h>
> #include <stdio.h>
> 
> #define MEG       (1024 * 1024)
> #define STACKSIZE (MEG * 2)
> 
> int main()
> {
>         int param = 0, i = 0;
>         LPFIBER_START_ROUTINE saddr = NULL;
>         LPVOID foo;
> 
>         ConvertThreadToFiber(&param);
> 
>         while(++i) {
> 
>                 foo = CreateFiber(STACKSIZE, saddr, &param);
> 
>                 if(foo != NULL)
>                         printf("Created fiber 0x%08x.\n", foo);
>                 else {
>                         printf("Failed to create fiber at iteration %d;\n"
>                                 "stack space allocated: %dMB\n",
>                                 i, (STACKSIZE * i) / MEG);
>                         break;
>                 }
>         }
> 
>         return 0;
> }
> 
> ---
> 
> The results are definitely interesting. The runtime code generated by VC++
> (6.0) and BCB (bcc32.exe v5.5) will allow stack allocation by fibers until
> total stack space allocated hits the free memory limit on the system.
> 
> Whereas with gcc, no matter how small you make your stack, the maximum
> number of fibers will be 52. (If it was 42, I'd suspect somebody was taking
> the Hitchhiker's Guide to the Galaxy a wee bit too seriously.)
> 
> All in all, I don't think this is a bug in the library. It definitely looks
> like a hard limit on the number of stacks/fibers you can allocate --
> something of the sort Danny Smith proposed, but "harder". Without looking at
> the source, I doubt anybody would be able to pin it down (unless they happen
> to be the gcc maintainer).
> 
> You might want to ask on the gcc mailing list(s).
> 
> Cheers,
> 
>   -- danp

Try changing the stacksize when build the app,
eg:

gcc -Wl,--stack,0x1000000 fiber.c

With this setting  I can get 126 fibers

As I set the app stack reserve smaller and smaller, I can create more fibers.
 
gcc -Wl,--stack,0x100000 fiber.c gets me 196 fibers (each with a 1 MB stack
reserve

and so on.

Hmmm, Does CreateFiber implicitly create a thread with a default (set by main
thread) stack reserve and then convert it to a fiber?


danny

> 


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