Re: [Boost-users] shared memory overhead

From: Jason Sachs (jmsachs_at_[hidden])
Date: 2008-05-30 09:56:45

>
> Modern systems use on demand allocation.  I.e. you can allocate a (f.ex.)
> 32 MB
> SHM chunk, but the actual resource usage (RAM) will correspond to what you
> actually use.  For example:
>
> 0   1M                 32M
> [****|..................]
>   |           |
>   |           +- unused part
>   |
>   +- used part of the SHM segment
> As long as your program does not touch (neither reads nor writes) the
> unused part,
> the actual physical memory usage will be 1M + small amount for page tables
> (worst
> case: 4kB of page tables for 4MB of virtual address space).  This is at
> least how
> SYSV SHM works on Solaris 10 (look up DISM - dynamic intimate shared
> memory); I
> would expect it to work in the same way on new linux kernels too.  I'm not
> sufficiently acquainted with NT kernel to be able to comment on it.

Interesting. I hadn't thought about that.

I tried a test program (running on Windows XP), and had a number of separate
processes each allocate a new managed_windows_shared_memory object (with a
different name for each process) with size 2^29 bytes (=512MB). I'm not
exactly sure what resources it allocates; using TaskInfo to view resource
usage, each process's "virtual KB" usage goes up by 512MB, but its "working
set KB" usage doesn't increase until I actually allocate memory within the
shared memory segment.

Sounds good, but the 6th one of these failed and I got a warning saying my
system was low on virtual memory. So it sounds like there is a 4GB total
system limit for WinXP even for just reserving virtual address space --
which seems silly since each process should have its own address space and
therefore as long as I don't actually allocate the memory, and each
process's reserved address space doesn't exceed 2^32 (or 2^31 or whatever
the per-process limit is), I should be able to reserve an unlimited amount
of total address space. No can do. :(

So strategy #1 of being profligate in choosing shared memory segment size
fails on WinXP; there's a significant resource cost even if you don't
actually allocate any memory. Drat.