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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Aptos",sans-serif">Not sure if this is too trivial for this group\u2026 but as a performance person on AIX I generally look to see how often is the sweeping hand coming by and how much RAM is it actually
collecting\u2026<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Aptos",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Aptos",sans-serif">On Linux you want to watch with something like this:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Aptos",sans-serif">watch -n1 'grep -E "pgfree|pgsteal|pgscan|nr_free_pages" /proc/vmstat'<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Aptos",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Aptos",sans-serif">To find out how much pressure your server is under you want to look to see how many page scans are being performed (is counter incrementing quickly)\u2026 that means the system WANTS
memory, versus how large are the page steals how much memory did it get\u2026<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Aptos",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Aptos",sans-serif">Typically, you\u2019d divide the steals by the scans to come up with a pressure ratio.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Aptos",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Aptos",sans-serif">If your server is using ALL of its RAM say for cache hits etc, that can be efficient, so long as the ratio is low to how much RAM it demands and how often is it trying to get
free memory. If memory is just lazily being occupied the inactive page count will be high, and as soon as a scan demands memory it will free a TON of memory. But if the memory is marked active, and the scan demands memory it won\u2019t be able to free memory,
so you\u2019ll end up with many scans and much smaller freed memory.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Aptos",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Aptos",sans-serif">Hope that helps.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Aptos",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Aptos",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> gpfsug-discuss <gpfsug-discuss-bounces@gpfsug.org>
<b>On Behalf Of </b>Jonathan Buzzard<br>
<b>Sent:</b> Monday, February 16, 2026 8:35 AM<br>
<b>To:</b> gpfsug-discuss@gpfsug.org<br>
<b>Subject:</b> Re: [gpfsug-discuss] swapped_warn event<o:p></o:p></span></p>
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<pre style="white-space:pre-wrap"><span style="font-size:12.0pt;font-family:"Arial",sans-serif">On 16/02/2026 11:43, Jan-Frode Myklebust wrote:<o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">> <o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">> It seems like someone thinks that linux servers should never use any<o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">> swap. This swapped_warn triggers if you've used more than 50 MB of<o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">> swap space. I find this silly..<o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">That's a polite way of putting it.<o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">Anecdotally, if memory starts to get tight Linux will preventatively <o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">push pages to swap "just in case", and might never actually use it. <o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">Consequently having a warning if more than 50MB of swap space is used is <o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">as useful as a chocolate teapot.<o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">If you are going to warn about swap being used, then it needs to be <o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">because the kernel was actually shuffling memory between disk and RAM <o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">not because it pushed some pages to disk just in case.<o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">On an HPC system where the compute nodes are frequently near maximum RAM <o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">usage all I unsurprisingly have a load of spurious warnings.<o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">> <o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">> You can tune it using something like "mmchconfig<o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">> mmhealth-gpfs-swap_alert_threshold_kb=2000000 --force", but I wouldn't<o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">> want to pollute my config with such settings.. Maybe just disable it<o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">> using "mmhealth event hide swapped_warn".<o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">> <o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">I am debating having a script automatically run "swapoff -a ; swapon -a" <o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">on the nodes if these warnings are seen :-)<o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">JAB.<o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">-- <o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">Jonathan A. Buzzard Tel: +44141-5483420<o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">HPC System Administrator, ARCHIE-WeSt.<o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">University of Strathclyde, John Anderson Building, Glasgow. G4 0NG<o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">_______________________________________________<o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">gpfsug-discuss mailing list<o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif">gpfsug-discuss at gpfsug.org<o:p></o:p></span></pre>
<pre><span style="font-size:12.0pt;font-family:"Arial",sans-serif"><a href="https://urldefense.com/v3/__http:/gpfsug.org/mailman/listinfo/gpfsug-discuss_gpfsug.org__;!!F9svGWnIaVPGSwU!o15h3llP_t-xAwde_PAYRCF1VYCK7qhlPXsK1TCZNCjGQSiX71ae4hsr_lQQzfzDbWX_iltIVIDP05jTcSf95e2pUMfriO6pIymIMw$">https://urldefense.com/v3/__http://gpfsug.org/mailman/listinfo/gpfsug-discuss_gpfsug.org__;!!F9svGWnIaVPGSwU!o15h3llP_t-xAwde_PAYRCF1VYCK7qhlPXsK1TCZNCjGQSiX71ae4hsr_lQQzfzDbWX_iltIVIDP05jTcSf95e2pUMfriO6pIymIMw$</a><o:p></o:p></span></pre>
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