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From: alan@shodha.enet.dec.com ( Alan's Home for Wayward Notes File.)
Newsgroups: comp.unix.ultrix
Subject: Re: A question about swap
Summary: Interleaved swap.
Keywords: swap
Message-ID: <3390@shodha.enet.dec.com>
Date: 16 Jun 91 07:01:04 GMT
References: <1991Jun14.184609.21178@mlb.semi.harris.com> <22486@cbmvax.commodore.com>
Organization: Digital Equipment Corp. - Colorado Springs, CO.
Lines: 42

In article <22486@cbmvax.commodore.com>, grr@cbmvax.commodore.com (George Robbins) writes:
> In article <1991Jun14.184609.21178@mlb.semi.harris.com> dcb@dave.mis.semi.harris.com writes:
> > [ A previous customer asked about how to best configure more
> >   page/swap space. ]
> 
> Supposedly the Berkeley code does distribute swap usage over the multiple
> drives in a way that improves performance.

	Yes it does.  You can demonstrate this by running iostat(1)
	or Monitor at the time you're doing a:

		# cp /dev/drum /dev/null
	
> 
> Most the requirement for large swaps spaces stems from the problem that
> Ultrix allocates swap space before it actually needs it.

	Actually ULTRIX V4.0 and later "reserves" the space at process 
	startup.  It doesn't get allocated until used.  The reservation 
	is necessary to avoid deadlocks and the harder problem of how 
	to cope with them.  Some systems cope by letting you dynamically 
	add space as needed.  Others cope by killing the offending process.
> 
> [ George discusses the performance aspects of the problem. ]

	If you don't page/swap (much) then the performance doesn't
	matter much.  If you do page/swap much, then you need more
	memory (up to a point).  If you do have to the pageing/swapping
	I/O then use the /dev/drum trick mentioned above.  It's hard
	to determine page-out performance this way, but you can see
	which is better for page-in.

	Also consider playing around with the configuration parameter
	"swapfrag".  It determines the I/O size for some things.  If
	you're doing a lot, then increaseing swapfrag may help.
> 
> -- 
> George Robbins - now working for,     uucp:   {uunet|pyramid|rutgers}!cbmvax!grr


-- 
Alan Rollow				alan@nabeth.cxn.dec.com