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<p>This seems quite reasonable to me.<br>
Not sure about the cost of implementing it (and the feasability of
it if/when merge-trains arrive).</p>
<p>Andreas<br>
</p>
<div class="moz-cite-prefix">Am 21/02/2021 um 21:31 schrieb Richard
Eisenberg:<br>
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cite="mid:010f0177c64a7c40-97040c50-92b1-4ad7-841d-8055824a8de2-000000@us-east-2.amazonses.com">
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<div class="">On Feb 21, 2021, at 11:24 AM, Ben Gamari <<a
href="mailto:ben@well-typed.com" class=""
moz-do-not-send="true">ben@well-typed.com</a>> wrote:</div>
<br class="Apple-interchange-newline">
<div class=""><span style="caret-color: rgb(0, 0, 0);
font-family: Menlo-Regular; font-size: 11px; font-style:
normal; font-variant-caps: normal; font-weight: normal;
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0px; text-transform: none; white-space: normal;
word-spacing: 0px; -webkit-text-stroke-width: 0px;
text-decoration: none; float: none; display: inline
!important;" class="">To mitigate this I would suggest
that we allow performance test failures</span><br
style="caret-color: rgb(0, 0, 0); font-family:
Menlo-Regular; font-size: 11px; font-style: normal;
font-variant-caps: normal; font-weight: normal;
letter-spacing: normal; text-align: start; text-indent:
0px; text-transform: none; white-space: normal;
word-spacing: 0px; -webkit-text-stroke-width: 0px;
text-decoration: none;" class="">
<span style="caret-color: rgb(0, 0, 0); font-family:
Menlo-Regular; font-size: 11px; font-style: normal;
font-variant-caps: normal; font-weight: normal;
letter-spacing: normal; text-align: start; text-indent:
0px; text-transform: none; white-space: normal;
word-spacing: 0px; -webkit-text-stroke-width: 0px;
text-decoration: none; float: none; display: inline
!important;" class="">in marge-bot pipelines. A slightly
weaker variant of this idea would</span><br
style="caret-color: rgb(0, 0, 0); font-family:
Menlo-Regular; font-size: 11px; font-style: normal;
font-variant-caps: normal; font-weight: normal;
letter-spacing: normal; text-align: start; text-indent:
0px; text-transform: none; white-space: normal;
word-spacing: 0px; -webkit-text-stroke-width: 0px;
text-decoration: none;" class="">
<span style="caret-color: rgb(0, 0, 0); font-family:
Menlo-Regular; font-size: 11px; font-style: normal;
font-variant-caps: normal; font-weight: normal;
letter-spacing: normal; text-align: start; text-indent:
0px; text-transform: none; white-space: normal;
word-spacing: 0px; -webkit-text-stroke-width: 0px;
text-decoration: none; float: none; display: inline
!important;" class="">instead only allow performance
*improvements*. I suspect the latter</span><br
style="caret-color: rgb(0, 0, 0); font-family:
Menlo-Regular; font-size: 11px; font-style: normal;
font-variant-caps: normal; font-weight: normal;
letter-spacing: normal; text-align: start; text-indent:
0px; text-transform: none; white-space: normal;
word-spacing: 0px; -webkit-text-stroke-width: 0px;
text-decoration: none;" class="">
<span style="caret-color: rgb(0, 0, 0); font-family:
Menlo-Regular; font-size: 11px; font-style: normal;
font-variant-caps: normal; font-weight: normal;
letter-spacing: normal; text-align: start; text-indent:
0px; text-transform: none; white-space: normal;
word-spacing: 0px; -webkit-text-stroke-width: 0px;
text-decoration: none; float: none; display: inline
!important;" class="">would get most of the benefit, while
eliminating the possibility that a</span><br
style="caret-color: rgb(0, 0, 0); font-family:
Menlo-Regular; font-size: 11px; font-style: normal;
font-variant-caps: normal; font-weight: normal;
letter-spacing: normal; text-align: start; text-indent:
0px; text-transform: none; white-space: normal;
word-spacing: 0px; -webkit-text-stroke-width: 0px;
text-decoration: none;" class="">
<span style="caret-color: rgb(0, 0, 0); font-family:
Menlo-Regular; font-size: 11px; font-style: normal;
font-variant-caps: normal; font-weight: normal;
letter-spacing: normal; text-align: start; text-indent:
0px; text-transform: none; white-space: normal;
word-spacing: 0px; -webkit-text-stroke-width: 0px;
text-decoration: none; float: none; display: inline
!important;" class="">large regression goes unnoticed.</span></div>
</blockquote>
</div>
<br class="">
<div class="">The value in making performance improvements a test
failure is so that patch authors can be informed of what they
have done, to make sure it matches expectations. This need can
reasonably be satisfied without stopping merging. That is, if
Marge can accept performance improvements, while (say) posting
to each MR involved that it may have contributed to a
performance improvement, then I think we've done our job here.</div>
<div class=""><br class="">
</div>
<div class="">On the other hand, a performance degradation is a
bug, just like, say, an error message regression. Even if it's a
combination of commits that cause the problem (an actual
possibility even for error message regressions), it's still a
bug that we need to either fix or accept (balanced out by other
improvements). The pain of debugging this scenario might be
mitigated if there were a collation of the performance wibbles
for each individual commit. This information is, in general,
available: each commit passed CI on its own, and so it should be
possible to create a little report with its rows being perf
tests and its columns being commits or MR #s; each cell in the
table would be a percentage regression. If we're lucky, the
regression Marge sees will be the sum(*) of the entries in one
of the rows -- this means that we have a simple agglomeration of
performance degradation. If we're less lucky, the whole will not
equal the sum of the parts, and some of the patches interfere.
In either case, the table would suggest a likely place to look
next.</div>
<div class=""><br class="">
</div>
<div class="">(*) I suppose if we're recording percentages, it
wouldn't necessarily be the actual sum, because percentages are
a bit funny. But you get my meaning.</div>
<div class=""><br class="">
</div>
<div class="">Pulling this all together:</div>
<div class="">* I'm against the initial proposal of allowing all
performance failures by Marge. This will allow bugs to
accumulate (in my opinion).</div>
<div class="">* I'm in favor of allowing performance improvements
to be accepted by Marge.</div>
<div class="">* To mitigate against the information loss of Marge
accepting performance improvements, it would be great if Marge
could alert MR authors that a cumulative performance improvement
took place.</div>
<div class="">* To mitigate against the annoyance of finding a
performance regression in a merge commit that does not appear in
any component commit, it would be great if there were a tool to
collect performance numbers from a set of commits and present
them in a table for further analysis.</div>
<div class=""><br class="">
</div>
<div class="">These "mitigations" might take work. If labor is
impossible to produce to complete this work, I'm in favor of
simply allowing the performance improvements, maybe also filing
a ticket about these potential improvements to the process.</div>
<div class=""><br class="">
</div>
<div class="">Richard</div>
<br>
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