question about fopen fd limit

Fri Dec 23 10:17:50 UTC 2016

Ok. I know.
There are two possible solutions:

  1.  Quick solution for short term: modify short to int in libc by yourself, buildworld and installworld. Pushing to modify libc may take a long time, especially only few people encounter this issue. You’d better send email to freebsd-current to confirm whether they accept your suggestion.
  2.  Work around: You can first reserve a series of fd before opening TCP connections. For example, invoke open(“/dev/null”) for 10000 times to get 10000 fds. Those fd values are small enough to be held by “short”. After that, start TCP connections. Once you need to fopen a file, please call open(“xxx”) instead, and then use dup2(old_fd, new_fd) to exchange the two fd. The old_fd value is the one obtained by open(“xxx”), and new_fd is one in your reserved fd fields, and next please use fdopen(fd, mode). Here, you have to manage the reserved fds by yourself including open/close.

In my eyes:

  1.  is the quick method, and there is no modifications in your logic.
  2.  Needs you to maintain the reserved consecutive fields for fd by yourself, which increased the complexity of your logic.

Thanks
Hongjiang Zhang

From: 盛慧华 [mailto:hhsheng at corp.netease.com]
Sent: Friday, December 23, 2016 11:02 AM
To: Hongjiang Zhang <honzhan at microsoft.com>; freebsd-net <freebsd-net at freebsd.org>
Subject: Re: RE: question about fopen fd limit

hi all,

  not map  TCP to FILE, you misunderstanding my meaning~

  for example, if my server tcp already holds 32000 connection
  fopen only has 767 fd to use

  the problem has no bussiness with tcp fd, BUT fopen ...

  in some particular situlations , my server will open 1k+ FILE , that will exceed the fileno limit, and overflow occur
  my server can't open any file more ,that's the problem ~

  so i felt if bsd official could change FILE struct's fileno to a UNSIGNED SHORT that may be an effecient and convenient solution just for my case ?
  UNSIGNED SHORT fileno is enough for me, and i don't wanna change a lot of FILE function that take FILE * as its argument ~

  Thank you ~~~

                                    winson sheng

________________________________
winson sheng

From: Hongjiang Zhang<mailto:honzhan at microsoft.com>
Date: 2016-12-23 10:17
To: 盛慧华<mailto:hhsheng at corp.netease.com>; freebsd-net<mailto:freebsd-net at freebsd.org>
Subject: RE: question about fopen fd limit
Why do you need to map TCP fd to FILE?

It is difficult to modify FILE structure. If it is possible, let us figure out some new designs to meet your requirement.

-----Original Message-----
From: owner-freebsd-net at freebsd.org<mailto:owner-freebsd-net at freebsd.org> [mailto:owner-freebsd-net at freebsd.org] On Behalf Of ???
Sent: Thursday, December 22, 2016 11:57 PM
To: freebsd-net <freebsd-net at freebsd.org<mailto:freebsd-net at freebsd.org>>
Subject: question about fopen fd limit

hi all,

   hi~
   we are from Chinese Game Develop Corp, Netease.
   and One of our product using FreeBsd as its OS platform.
   This Game has Millions of players online , and Each Server may holds 25000+ tcp connection at the same time.Thanks to BSD and kqueue :)

   for example, it's one of our server , netstat cmd to list connections overall...
   netstat -an | grep 13396 (it's our listening port) | wc -l
   23221

    recently we do some performance optimize and promote this connect limit to 28000+ or 30000+.
  But we find Freebsd has a limit that this huge online number will take 28000+ fd, and bsd FILE * struct's
  fd only support to SHORT . such as ..

struct __sFILE {
...
short _file; /* (*) fileno, if Unix descriptor, else -1 */  ...

  so if our server want to fopen some file when we still hold this online number, the fd amount may easily exceed 32767, and fopen definitely return a err code. then the server will appear some fataly ERROR.

  we do a simple test and confirm this situation.

  then in fopen's code , we notice that we can use open to return a fd instread of fopen to avoid this overflow,
   as below

68 /*
1 * File descriptors are a full int, but _file is only a short.
2 * If we get a valid file descriptor that is greater than
3 * SHRT_MAX, then the fd will get sign-extended into an
4 * invalid file descriptor. Handle this case by failing the
5 * open.
6 */

  BUT ... so many c lib FILE series function needs a FILE * pointer as input argument, we can't convert all of them to fd, or it will be a rather suffering things to us.

  and even in BSD 10 , it seems this short limit still there , but other OS as debian , FILE strucnt's fileno is a int .

  we found an unoffical patch easily change this fileno to unsigned , but we are a very stready project, we can't afford the risk to use an unoffical patch.

  so, do you have any plan to change this fopen fd limit to UNSIGNED SHORT or int in the future ? ushort is enough for us .
  if you do , we are really glad and excited~~~~~~~if you don't ,it donen't matter,  plz give us a reply so that we may need to
  find some other plan to resolved this suffering thing.
  LoL, thank you !!!!!

yours sincerely
                                       winson sheng

winson sheng
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