Bug(s) with creating large numbers of sockets
Fri Nov 3 20:36:00 GMT 2017
why didn't you send this to cygwin-patches? Not much of a problem,
On Nov 3 15:39, Erik Bray wrote:
> After some playing around I found that I could make up to exactly 1365
> sockets and use them without error. At 1366 I get the error. A very
> strange and arbitrary number. It turns out this is limited in Cygwin
> by the array in fhandler_socket.cc:
> 496 /* Maximum number of concurrently opened sockets from all Cygwin processes
> 497 per session. Note that shared sockets (through dup/fork/exec) are
> 498 counted as one socket. */
> 499 #define NUM_SOCKS (32768 / sizeof (wsa_event))
> 510 static wsa_event wsa_events[NUM_SOCKS] __attribute__((section
> (".cygwin_dll _common"), shared));
> This choice for NUM_SOCKS is still seemingly small and pretty
> arbitrary, but at least it's a choice, and probably well-motivated.
This obviously dates back to pre-64 bit times. The idea at the time was
that the .cygwin_dll_common section fits into a single 64K allocation
chunk. Every section takes at least this 64K chunk, so we get away with
only one of them for this section.
Turns out, shared sections have a few issues so we even reduced the
usage and only kept stuff in which doesn't pose much of a problem.
Right now .cygwin_dll_common has a size of 0x8220 / 0x8200 of a max.
of 0x10000, so we have still lots of room and nothing really in need
of using a shared section.
> However, I think it's a problem that it's defined in terms of
> sizeof(wsa_event). On 32-bit Cygwin this is 16, so NUM_SOCKS is 2048
> (a less strange number), whereas on 64-bit Cygwin sizeof(wsa_event) ==
> 24 (due to sizeof(long) == 8, plus alignment), so we are limited
> to...1365 sockets.
> If we have to set a limit I would just hard-code it to 2048 exactly.
> I understand that the overhead associated with sockets in Cygwin
> probably limits us from having 10s of thousands (much less millions)
> and that's OK--I'm not trying to run some kind of C10K challenge on
> Cygwin :)
We only need another 0x220 bytes so we have a theoretical 0xfde0 / 64992
bytes or 2708 sockets for wsa_events on 64 bit.
So, yeah, I think you're right, 2048 is good number. As I wrote in the
comment preceeding NUM_SOCKS, these are 2048 independent sockets per
user session in parallel. Sockets created by dup or shared via
fork/exec only take one slot anyway. 2048 independent sockets per user
session should really suffice outside of testcases.
> The other problem, then, seems to be a bug in
> fhandler_socket::init_events(). It doesn't check the return value of
> search_wsa_event_slot(), which returns NULL if the wsa_events array is
> full (and the socket is not a shared socket). There's not a great
> choice here for error code, but setting ENOBUF seems like the best
Yep, bug. Thanks for catching.
> Please see attached patch.
Can you please send it to cygwin-patches with a bit of additional
comment (just a couple of words) why we choose 2048 here?
Corinna Vinschen Please, send mails regarding Cygwin to
Cygwin Maintainer cygwin AT cygwin DOT com
-------------- next part --------------
A non-text attachment was scrubbed...
Size: 819 bytes
Desc: not available
More information about the Cygwin