Re: a swap partition utilization question

["Bill J.Xu" <xujz@neusoft.com>]

Hi, Clements:

Thank you for your detailed explains first.

I use the linux box as a firewall, so it is highly loaded at any time.At most time, it's available physical RAM is less 2M, but SWAP partition is never used. This 
make me confused.



Bill


----- Original Message ----- 
From: "Glynn Clements" <glynn.clements@virgin.net>
To: "Bill J.Xu" <xujz@neusoft.com>
Cc: "chuck gelm net" <chuck@gelm.net>; <linux-admin@vger.kernel.org>
Sent: Wednesday, November 26, 2003 8:32 PM
Subject: Re: a swap partition utilization question


> 
> Bill J.Xu wrote:
> 
> > > > After booting my linux box,I add a swap partition using "swapon
> > > > /dev/hda5".but I found that swap partition was nerver used even
> > > > 2M physics memory left.
> > > > 
> > > > why?
> > > 
> > >  Perhaps swap memory will not be used until
> > > all physical memory has been consumed.
> > 
> > But I found that other linux box had used swap partition when many
> > physical RAM left.
> > 
> > root@FW_knl:/home/shiy# free
> >              total       used       free     shared    buffers     cached
> > Mem:        255320     247564       7756          0      15652     197560
> > -/+ buffers/cache:      34352     220968
> > Swap:       996020       1484     994536
> > root@FW_knl:/home/shiy# 
> 
> How long has the first box been up? And how intensive is memory demand
> on that box?
> 
> Swap won't be used at all until physical memory becomes close to being
> exhausted. However, after that point, even if more memory becomes
> available, data won't be moved back into RAM unless it is actually
> used.
> 
> There is often some data which will never be used again (e.g. memory
> which is used by persistent daemons, but only during the
> initialisation; the 6 getty processes running on tty1-tty6 which
> typically never get used on a system which is running X, etc). Such
> data is a strong candidate for being swapped out; once that happens,
> it will typically never be swapped in again.
> 
> So, when a system is booted, there will always be a period when no
> swap is used. If memory demand is low, that period may last
> indefinitely. OTOH, once memory demand has been sufficiently high to
> have required the use of swap, swap usage will probably never drop
> back to zero.
> 
> -- 
> Glynn Clements <glynn.clements@virgin.net>
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