Обсуждение: FD_SETSIZE on Linux?
Hi,
Thom
I noticed when trying to set pgbench's client count to a high number, I had to reduce it, and I found the maximum I can get away with is 1014. Any higher and I get:
invalid number of clients: 1015
I find this in pgbench.c:
#ifdef FD_SETSIZE
#define MAXCLIENTS (FD_SETSIZE - 10)
#else
#define MAXCLIENTS 1024
#endif
And FS_SETSIZE defined before it:
#ifdef WIN32
#define FD_SETSIZE 1024 /* set before winsock2.h is included */
#endif /* ! WIN32 */
... but apparently only if using Windows, which I'm not.
So it appears that MAXCLIENTS is being set to 1014 (1024 - 10), which looks like should only be the case on Windows.
I'm a bit confused here. Shouldn't my MAXCLIENTS be set to 1024?
Thom Brown <thom@linux.com> writes:
> I find this in pgbench.c:
> #ifdef FD_SETSIZE
> #define MAXCLIENTS (FD_SETSIZE - 10)
> #else
> #define MAXCLIENTS 1024
> #endif
FD_SETSIZE is supposed to be defined, according to the POSIX spec:
The <sys/select.h> header shall define the following symbolic constant, which shall have a value suitable for use
in#if preprocessing directives:
FD_SETSIZEMaximum number of file descriptors in an fd_set structure.
It looks like Linux sets it to 1024. On RHEL6, at least, I find this:
$ grep -r FD_SETSIZE /usr/include
/usr/include/linux/posix_types.h:#undef __FD_SETSIZE
/usr/include/linux/posix_types.h:#define __FD_SETSIZE 1024
...
/usr/include/sys/select.h:#define FD_SETSIZE __FD_SETSIZE
...
> #ifdef WIN32
> #define FD_SETSIZE 1024 /* set before winsock2.h is included */
> #endif /* ! WIN32 */
Windows probably hasn't got sys/select.h at all, so it may not provide
this symbol.
Interestingly, it looks like POSIX also requires <sys/time.h> to define
FD_SETSIZE. I wonder whether Windows has that header? It'd definitely
be better to get this symbol from the system than assume 1024 will work.
regards, tom lane
On 10 September 2014 00:21, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Thom Brown <thom@linux.com> writes:
> I find this in pgbench.c:
> #ifdef FD_SETSIZE
> #define MAXCLIENTS (FD_SETSIZE - 10)
> #else
> #define MAXCLIENTS 1024
> #endif
FD_SETSIZE is supposed to be defined, according to the POSIX spec:
The <sys/select.h> header shall define the following symbolic constant,
which shall have a value suitable for use in #if preprocessing directives:
FD_SETSIZE
Maximum number of file descriptors in an fd_set structure.
It looks like Linux sets it to 1024. On RHEL6, at least, I find this:
$ grep -r FD_SETSIZE /usr/include
/usr/include/linux/posix_types.h:#undef __FD_SETSIZE
/usr/include/linux/posix_types.h:#define __FD_SETSIZE 1024
...
/usr/include/sys/select.h:#define FD_SETSIZE __FD_SETSIZE
...
Ah yes, I have the same on Debian:
/usr/include/linux/posix_types.h:#undef __FD_SETSIZE
/usr/include/linux/posix_types.h:#define __FD_SETSIZE 1024
...
usr/include/x86_64-linux-gnu/sys/select.h:#define FD_SETSIZE __FD_SETSIZE
/usr/include/x86_64-linux-gnu/bits/typesizes.h:#define __FD_SETSIZE 1024
...
I didn't think to look beyond Postgres' code.
> #ifdef WIN32
> #define FD_SETSIZE 1024 /* set before winsock2.h is included */
> #endif /* ! WIN32 */
Windows probably hasn't got sys/select.h at all, so it may not provide
this symbol.
Interestingly, it looks like POSIX also requires <sys/time.h> to define
FD_SETSIZE. I wonder whether Windows has that header? It'd definitely
be better to get this symbol from the system than assume 1024 will work.
Okay, this now makes sense. It just takes the system value and reduces it by 10 to get the MAXCLIENTS value.
Thanks for the explanation.