Re: [Boost-users] shared memory overhead

From: Zeljko Vrba (zvrba_at_[hidden])
Date: 2008-05-30 13:53:34

On Fri, May 30, 2008 at 12:25:25PM -0400, Jason Sachs wrote:
> 
> >Could you maybe use a raw memory-mapped file instead,
> >and convert it to HDF5 off-line?
> 
> well, technically yes, but for robustness reasons I want to decouple
>
ok, i just wanted to know more about your problem.

> 
> Boost::interprocess to make the segment size equal to M. If I do this
> then my resource usage in the page file (or on disk if I use a
> memory-mapped file) is N*M which is much higher than I need. (I
>
Which is much higher than you need  on the average.  I fail to see why
having a 10GB, or even a 20GB, swap-file is a problem for you.  Too much
of a hassle to configure it on all workstations?

> 
> of these going on at once (though usually just one or two). On my own
> computer I have increased my max swap file size from 3GB to 7GB (so
> the hard limit is somewhat adjustable), though it didn't take effect
> until I restarted my PC. I'm going to be using my programs on several
> computers + it seems silly to have to go to this extent.
>
Ok, and you'll run your job on a machine with e.g. 1GB of swap[*], and this
particular instance will need 4GB of swap.  What will happen when the
allocation fails?  Note that growing the SHM segment in small chunks will
not help you with insufficient virtual memory, so you might as well allocate
M*N at once and exit immediately if the memory is not available.

[*] I'm using "swap" somewhat imprecisely to refer to total virtual memory 
(RAM + swap).

Next-best solution: use binary search to find the maximum size you can
allocate and use that instead of M*N.

Neither way is particularly friendly towards other processes on the machine
(I assumed that you were running the jobs on dedicated machines), but is 
least painful.  Which is least expensive: your time spent developing multi-
chunk SHM management or just allocating a big chunk and reconfiguring all
computers *once*?

(I'm sorry, I'm very pragmatic, and I don't seem to have enough info to really
understand why you're making such a fuss over the swap size issue.  I'm afraid
I can't offer you any further suggestions, since I consider this a non-problem
unless you have further constraints.)