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<TITLE>The Answer Guy 36:
Swap file on a RAM Disk
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<H4>"The Linux Gazette...<I>making Linux just a little more fun!</I>"</H4>
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<H4>By James T. Dennis,
<a href="mailto:answerguy@ssc.com">answerguy@ssc.com</a><BR>
Starshine Technical Services,
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<H3 align="left"><img src="../../gx/dennis/qbubble.gif" height="50" width="60"
alt="(?) " border="0">
Swap file on a RAM Disk
</H3>
<p><strong>From Mathieu Bouchard on Wed, 02 Dec 1998
</strong></p>
<!-- ::
Swap file on a RAM Disk
~~~~~~~~~~~~~~~~~~~~~~~
:: -->
<P><STRONG><IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
HEIGHT="28" WIDTH="50" BORDER="0"
>
Hi,
</STRONG></P>
<P><STRONG><FONT COLOR="#000066"><EM>
Some have even reported that using 100 or 200K RAM disk with a swap file
on it will dramatically improve the performance over using all of your
memory as straight RAM.
</EM></FONT></STRONG></P>
<P><STRONG>
Do you have any rational explication to this? I'm not a kernel expert, but
it makes no sense -- especially because AFAIK, Linux RAM disks are
swappable (and lazily-allocated), and mutual containment (in this context)
makes no sense;
</STRONG></P>
<BLOCKQUOTE><IMG SRC="../../gx/dennis/bbub.gif" alt="(!)"
HEIGHT="28" WIDTH="50" BORDER="0"
>
No. I don't have a rational explication or explanation
for this.
</BLOCKQUOTE>
<P><STRONG><IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
HEIGHT="28" WIDTH="50" BORDER="0"
>
but in the event that a RAM disk wouldn't be swappable, then, swapping
from RAM to RAM isn't anything more than a CPU hog and unnecessary
complexity -- it's a kind of Alice in Wonderland to me. It would make
sense if some compression was done while swapping, which would look like a
Macintosh RAMdoubler. But Linux has no such feature -- six months ago I
asked the Linux guys and they said that they didn't like the idea.
</STRONG></P>
<P><STRONG>
Is it possible that such a report would be gibberish? in which case I
would like you to get the precise facts and publish them. I think that
even though it is a detail, the Linux community doesn't deserve to have
anything done wrong. I'm not [bf]laming, I just want to correct a
situation.
</STRONG></P>
<P><STRONG>
matju
</STRONG></P>
<BLOCKQUOTE><IMG SRC="../../gx/dennis/bbub.gif" alt="(!)"
HEIGHT="28" WIDTH="50" BORDER="0"
>
However, I can make a guess. Many of the memory management
code paths may have to special case the situation where no
swap/paging space is available. The routines invoked to
handle this special case may result in a slow down when
no swap is available.
</BLOCKQUOTE>
<BLOCKQUOTE>
You're welcome to search the Linux-Kernel mailing list
archives yourself. You can also just try it (run some
tests with no swap space "mounted" and then run them
again with just a small swap file located on a small
RAM disk. I haven't actually tried this experiment,
so I made an effort to identify my statement as hearsay.
</BLOCKQUOTE>
<BLOCKQUOTE>
If you'd like to so some research on it --- you could
publish your results (probably in LG --- perhaps as a
2-cent tip or as a full article).
</BLOCKQUOTE>
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<H5 align="center"><a href="http://www.linuxgazette.com/ssc.copying.html"
>Copyright ©</a> 1999, James T. Dennis
<BR>Published in <I>The Linux Gazette</I> Issue 36 January 1999</H5>
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