Subject: Re: fsync performance hit on 1.6.1
To: Matt Thomas <>
From: Kamal R Prasad <>
List: tech-kern
Date: 07/10/2003 11:41:38
IMHO (in retrospect), if a system call does not provide any new feature 
(shm vs mmap), it needs to be deprecated/moved to user-space [using a 
common mechanism to share data efficiently]. 


Matt Thomas <>
Sent by:
07/09/2003 07:13 PM
        To: (NetBSD Kernel Technical Discussion 
        Subject:        Re: fsync performance hit on 1.6.1


At 09:26 AM 7/9/2003, Greg A. Woods wrote:
>If you know how IEEE standards committees work and you understand how
>much they (are supposed to) hate inventing new things, the fact that
>they invented shm_open and shm_unlink() suggests that some strong
>member(s) of the comittee were just completely and totally unwilling to
>allow for mmap() to work on all normal files and that the only way they
>would be happy with mmap() becoming the true standard shared memory
>interface was if it was required that the file descriptors it used be
>allocated by some special new function.
>You would think shared memory would be simpler to describe and discuss
>than something like message queues which have lots of fancy features,
>since it is, after all, just a chunk of memory storage that can be
>mapped into the address space of multiple processes.  However because of
>this strange use of mmap() and all the qualifiers they put on it for
>POSIX, folks like Bill Gallmeister in his O'Reilly "POSIX.4" book
>actually spend more pages describing shared memory and give all kinds of
>caveats about its use.  The SysV SHM API is trivial by comparison to 

I forwarded Bill Gallmeister your message, and he gave me the following
to use as a response:

And to comment on the email thread you forwarded:  hindsight's a beautiful
thing.  shm_open et al were designed to allow a trivial implementation 
mmap (or atop the Ludicrous Sys V Interface (TM)), but at the time we came 
with the standard, I believe it was only the rocket scientists at Sun who 
mmap--no one else had ponied up to the memory==file proposition and all 
implications.  There wasn't even an INTERNET back then, for Chrissake.  Al 
hadn't been BORN.  We wrote the damn standard using OIL LANTERNS.  Okay, 
I'm exaggerating a little.  It was a DECADE ago!

Matt Thomas               Internet:
3am Software Foundry      WWW URL:
Cupertino, CA             Disclaimer: I avow all knowledge of this message