From:	MERC::"uunet!CRVAX.SRI.COM!RELAY-INFO-VAX"  1-JUL-1992 09:39:57.70
To:	info-vax@kl.sri.com
CC:	
Subj:	Re: iozone on vax/vms is slow

In article <1992Jun30.141251.6408@dxcern.cern.ch>, michelotto@dxcern.cern.ch (Michele Michelotto) writes:
|> 
|> I tried IOZONE 1.15 on a vaxstation 4000/60 and 3100/76 with vms 5.5
|> and on a DS5000/200 with Ultrix 4.1 with different disk.
|> I've the impression that IOZONE has some problem on vms because I get
|> very poor figure.
<stuff deleted>

Actually, poor showing on VMS is due to the poor implementation of the write() function
in VAX C.  I originally wrote IOzone (about a year before V1.0 was published to the
outside world), while I was working in VMS Development at DEC on the VMS POSIX
project.  I wanted to compare the then-current performance of C I/O on VMS and ULTRIX
in order to see if the VMS POSIX file system would meet the  goal: at least 85% as fast as 
ULTRIX on the same model VAX (it isn't) and at least as fast as VAX C I/O (it is).  I never 
exceeded 25K bytes/second on writes on any of the VAXes I tested it on (big ones with
fast disks).  To verify that the bottleneck was in the C runtime library, I tested IOzone with
the GNU C compiler on VMS.   I don't have the exact figures anymore, but gcc was on the 
order of 4-5 times faster than VAX C on writes.    

VAX C write() does not seem to use the RMS per-process buffer caching code, but if I
remember correctly it does use RMS to open and do I/O to the file.  VMS, unlike
UNIX, does not have a system-wide file system cache.   At the $QIO level of VMS I/O, 
writes go directly to disk.   I could have written IOzone to use the VMS-specific $QIO
system calls, but my objective was to have a portable program.   In fairness to VMS, if
you use POSIX synchronous I/O by opening the file with the O_SYNC modifier, then
VMS has a much better showing versus the UNIX platforms.    If you are developing a
VMS-specific application, I would consider using asynchronous virtual file I/O calls 
using $QIO with ASTs to get maximum performance, this is one of the strong points
of VMS.   However, if you are interested in running portable C applications on VMS,
be aware that the performance of write() with VAX C is poor.
-----------------------------------------------------------------------|
Bill Norcott			GUARDIAN POSIX project
Tandem Computers, Inc.		
10600 N. Tantau Avenue		PHONE:  (408) 285-3253
Cupertino, CA   95014			EMAIL: norcott_bill@tandem.com