Re: Calculating the required value of "ulimit -q"

["Iñaki Baz Castillo" <ibc@aliax.net>]

El Viernes, 15 de Enero de 2010, Eric Wong escribió:

> I don't think it makes much sense since it doesn't (and can't) take
> other queues created by the user into account.  Even if mqueuefs is
> mounted under Linux, ownerships can change (and the Linux kernel tracks
> sizes for the user that created them, and this doesn't change if
> ownership changes).
> 
> In my experience, trying to be "helpful" with error handling/recovery
> that may end up misleading is worse than just showing the error for the
> user to handle.  I consider the best way is just to return the errno so
> the user can consult OS-specific documentation and investigate the issue.
> This is also a "startup" issue that should be quickly noticed/diagnosed
> when an application is launched.
> 
> Since I realize POSIX message queues are new territory to a lot of folks
> (myself included), I've been meaning to get started on an FAQ RDoc that
> I can distribute with the gem/tarball and add to the website.

Hi Eric.

Perhaps I explained wrong what I mean. I already deal with all the possible 
exceptions posix_mq raises (no enough memory, inavalid values due to user 
permissions, reusage of existing mqueues so atributes cannot be changed and 
so).

What I propose is really simple: a method to determine tha amount of bytes a 
posix queue would require, and it just depends on 3 factors:
- maxmsg
- msgsize
- 32 or 64 bits OS

So the only I propose (read please my last mail in which I attach a working 
code for posix_mq_ext.c) is a method to know how many bytes a queue would 
require, so the API looks like this:

  POSIX_MQ::size(msgsize, maxmsg)

example:

  irb> POSIX_MQ::size(1024, 5000)
  5160000

Why this could be useful? Imagine your program creates a mqueue with a 
required size. Before trying to create the queue it could read the user 
rlimits for mqueues and try to increase them. The following is a fragment of 
my application:
  

  MQUEUE_SIZE = POSIX_MQ::size(1024, 5000)
  
  if ( current_rlimit = Process.getrlimit(12)[1] ) < MQUEUE_SIZE
    log.info "incrementing rlimits for posix msgqueue (currently " +
             "#{current_rlimit} bytes) to #{MQUEUE_SIZE} bytes"
    begin
      Process.setrlimit(12, MQUEUE_SIZE)
    rescue Errno::EPERM
      log.warn "the current user has no permissions to increase " +
			 "rlimits for posix msgqueue"
    end
  end

  MQUEUE_ATTR          = POSIX_MQ::Attr.new
  MQUEUE_ATTR.maxmsg   = 5000
  MQUEUE_ATTR.msgsize  = 1024
  MQUEUE_ATTR.flags    = 0
  MQUEUE_ATTR.curmsgs  = 0

  begin
    @mq = POSIX_MQ.new MQUEUE_NAME, IO::RDONLY | IO::CREAT | IO::EXCL, 
                       00620, MQUEUE_ATTR
  rescue Errno::ENOSYS => e
    fatal "the kernel has no support for posix queue messages, enable it" 
  rescue Errno::EEXIST
    log.warn "posix message queue already exits so cannot change its " +
             "attributes"
    begin
      @mq = POSIX_MQ.new MQUEUE_NAME, IO::RDONLY
    rescue Errno::EACCES => e
      fatal "cannot open the existing posix message due to user permissions"
    end
  rescue Errno::EINVAL
    log.warn "cannot set posix message queue attributes due to user " + 		      
             "permissions, using system default values"
    @mq = POSIX_MQ.new MQUEUE_NAME, IO::RDONLY | IO::CREAT, 00620
  rescue Errno::ENOMEM => e
    fatal "cannot create posix message queue due to current user's rlimits"
  end


It works very well :)


-- 
Iñaki Baz Castillo <ibc@aliax.net>