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<p>OK, <i>Oh Smarter Colleagues</i>, the challenge to you is to
say if there is a "natural" place to capture state changes to
get the data we want, and if so, is it common or similar enough
between drivers to be worthy of attention?<br>
</p>
<p>--dave<br>
</p>
<div class="moz-cite-prefix">On 2021-06-09 9:15 a.m., Dave Taht
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:950B8EAF-90B9-41A6-951D-91821F591D41@teklibre.net">
<br class="">
<div><br class="">
<blockquote type="cite" class="">
<div class="">Begin forwarded message:</div>
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<span style="font-family: -webkit-system-font, Helvetica
Neue, Helvetica, sans-serif; color:rgba(0, 0, 0, 1.0);"
class=""><b class="">From:
</b></span><span style="font-family:
-webkit-system-font, Helvetica Neue, Helvetica,
sans-serif;" class="">David Collier-Brown <<a
href="mailto:davecb.42@gmail.com" class=""
moz-do-not-send="true">davecb.42@gmail.com</a>><br
class="">
</span></div>
<div style="margin-top: 0px; margin-right: 0px;
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<span style="font-family: -webkit-system-font, Helvetica
Neue, Helvetica, sans-serif; color:rgba(0, 0, 0, 1.0);"
class=""><b class="">Subject:
</b></span><span style="font-family:
-webkit-system-font, Helvetica Neue, Helvetica,
sans-serif;" class=""><b class="">Microstate Accounting
and the Nyquist problem</b><br class="">
</span></div>
<div style="margin-top: 0px; margin-right: 0px;
margin-bottom: 0px; margin-left: 0px;" class="">
<span style="font-family: -webkit-system-font, Helvetica
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class=""><b class="">Date:
</b></span><span style="font-family:
-webkit-system-font, Helvetica Neue, Helvetica,
sans-serif;" class="">June 9, 2021 at 4:44:14 AM PDT<br
class="">
</span></div>
<div style="margin-top: 0px; margin-right: 0px;
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<span style="font-family: -webkit-system-font, Helvetica
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class=""><b class="">To:
</b></span><span style="font-family:
-webkit-system-font, Helvetica Neue, Helvetica,
sans-serif;" class="">Dave Taht <<a
href="mailto:davet@teklibre.net" class=""
moz-do-not-send="true">davet@teklibre.net</a>><br
class="">
</span></div>
<div style="margin-top: 0px; margin-right: 0px;
margin-bottom: 0px; margin-left: 0px;" class="">
<span style="font-family: -webkit-system-font, Helvetica
Neue, Helvetica, sans-serif; color:rgba(0, 0, 0, 1.0);"
class=""><b class="">Cc:
</b></span><span style="font-family:
-webkit-system-font, Helvetica Neue, Helvetica,
sans-serif;" class="">Dave Collier-Brown <<a
href="mailto:dave.collier-brown@indexexchange.com"
class="" moz-do-not-send="true">dave.collier-brown@indexexchange.com</a>><br
class="">
</span></div>
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<span style="font-family: -webkit-system-font, Helvetica
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class=""><b class="">Reply-To:
</b></span><span style="font-family:
-webkit-system-font, Helvetica Neue, Helvetica,
sans-serif;" class=""><a
href="mailto:davecb@spamcop.net" class=""
moz-do-not-send="true">davecb@spamcop.net</a><br
class="">
</span></div>
<br class="">
<div class="">
<div class="">
<p class="">A million years ago (roughly around Solaris
9), Sun was suffering from the same problems in
measuring their dispatcher as you are with "sloshing".</p>
<p class="">A CPU would be 100% busy in one microsecond,
10% busy in the next gazillion, and the average CPU
utilization for our sample period would be
<i class="">maybe</i> 10.1, if the sampler happened to
sample right when the spike was happening.</p>
<p class="">This was utterly useless for things like the
fair-share scheduler, so it got fixed in Solaris 10,
by having the dispatcher record the time a process
(well, kernel thread) had spent in a state when the
state changed.<br class="">
</p>
<p class="">Initially "microstate accounting" could be
toggled on and off, but the branch-around cost more
time than always doing the calculation (as discovered
by my mad friend Fred) and the kernel folks left it
on. It's on to this day.</p>
<p class="">In Simon Sundberg's talk, the opportunity to
measure occurs every 1,000 packets, when a suitable
timestamp is provided. While the eBPF program can look
at every packet and do after-the-fact book-keeping in
a map, that's only good if the phenomenon you're
measuring is persistent enough that it's around for
~2,000 packets.</p>
<p class="">I'm going to suggest that the right place to
record the information you want is right where the
event happens. Preferably in c code, as performance
is easy to mess up, but perhaps with an eBPF mechanism
to export it.</p>
<p class="">In previous Solaris work, I reliably found
that exporting kstats was a darn sight harder than
collecting them, and in Eric's blog post[1] he notes
that converting time is expensive and best done long
after collecting, when someone wanted to read the
data.</p>
<p class="">There was an effort to do kstats in
Linux[2], but it had supposedly poor performance, and
actual trouble when the clock frequency changed.<br
class="">
</p>
<p class="">Is there, in your opinion, a "natural" place
to capture state changes to get the data you want, and
if so, is it common or similar enough between drivers
to be worthy of attention?</p>
<p class="">--dave<br class="">
</p>
<p class=""><br class="">
</p>
<p class="">References:</p>
<ol class="">
<li class="">Solaris: <a
class="moz-txt-link-freetext"
href="https://can01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fdtrace.org%2Fblogs%2Feschrock%2F2004%2F10%2F13%2Fmicrostate-accounting-in-solaris-10%2F&data=04%7C01%7C%7C7f7cd5aab2ca42e2e7e908d92d25e27f%7Cb07c069022b843668d8d7b845d088e18%7C1%7C0%7C637590463000477252%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=fdZDOtRCcBk%2BO1ksiTOSU%2FltR8IMwueHyj0kQG4UkHw%3D&reserved=0"
originalsrc="http://dtrace.org/blogs/eschrock/2004/10/13/microstate-accounting-in-solaris-10/"
shash="r++8CBJzd3EiHdQD4yln/JywHEgcQZcRkADLM58pY3y4GIQM79qWqmhLCC/gRJFmrZMcTsRTXYWsjJvwqLaUNTcyJvdbeC+sFPghSwQwf0ml5RWpT/hdeHE62U3EYo3yqhk0XWHHRmrDgD5wIcJPF8LNpbygu6zdFrcp5AUtudE="
moz-do-not-send="true">
http://dtrace.org/blogs/eschrock/2004/10/13/microstate-accounting-in-solaris-10/</a>
<br class="">
</li>
<li class="">A failing Linux effort: <a
class="moz-txt-link-freetext"
href="https://can01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flwn.net%2FArticles%2F127296%2F&data=04%7C01%7C%7C7f7cd5aab2ca42e2e7e908d92d25e27f%7Cb07c069022b843668d8d7b845d088e18%7C1%7C0%7C637590463000487248%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=uN0gq8vi0GJHMPpjKVYRjX6G5nQOc%2BugxUwUEk3%2BWJ8%3D&reserved=0"
originalsrc="https://lwn.net/Articles/127296/"
shash="NeRrtMZSW/w5Z16EVp+L4mx76CnciKROysvnuvIYMwhpmHy1kPIu8UIvlSNPVJr3mrf6T8eg/A9RZFdY3ToSPDwOK9AXprKGFxF5bfPklETFT2/wyZDMQg+32h2Au2fNqlAk1p20ndsJ2B3+iEmm08ARfHCVl7c8Z3RpgKoan60="
moz-do-not-send="true">
https://lwn.net/Articles/127296/</a>, <a
class="moz-txt-link-freetext"
href="https://can01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fsourceforge.net%2Fprojects%2Fmicrostate%2F&data=04%7C01%7C%7C7f7cd5aab2ca42e2e7e908d92d25e27f%7Cb07c069022b843668d8d7b845d088e18%7C1%7C0%7C637590463000497242%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=iMNi40Pl9hMmd1h7WrLFP5jmHQ60mJl7zehhO8miJv4%3D&reserved=0"
originalsrc="https://sourceforge.net/projects/microstate/"
shash="EaATi/ge264upYiz/waLkJTFhNT5ya2c8AUQqq5JoVbfuWm//GeW5inxQP8rULFqg7ezIt8agie84EjTPfOKSsmTlrVx7IVrdUIQdV3qSc0D2gNrGzTCYSEeYSd1AQhTNbTx3c8CCg2k4xiUArgx1w5vfMhPymyvv501lHYtKH4="
moz-do-not-send="true">
https://sourceforge.net/projects/microstate/</a><br
class="">
</li>
</ol>
<pre class="moz-signature" cols="72">--
David Collier-Brown, | Always do right. This will gratify
System Programmer and Author | some people and astonish the rest
<a class="moz-txt-link-abbreviated" href="mailto:davecb@spamcop.net" moz-do-not-send="true">davecb@spamcop.net</a> | -- Mark Twain
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