<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div dir="ltr"></div><div dir="ltr">I have to agree a bit with Arnaud. I don’t like the idea that I have to “reason about” these issues. Imagine a server service that encodes video. Highly cpu bound. But then a need to emit heartbeats to keep clients alive. I have to know “put heartbeat requests on their own native thread” because too many client encoding requests will cause them not to be sent. Or even more simply - a very short encoding request takes a very long time because it is blocked by other long running requests that got there first. FIFO doesn’t lead to the best user experience at times. </div><div dir="ltr"><br></div><div dir="ltr">With timeslicing and fairness that can be avoided. </div><div dir="ltr"><br></div><div dir="ltr"><br><blockquote type="cite">On Jan 6, 2023, at 9:50 AM, Ron Pressler <ron.pressler@oracle.com> wrote:<br><br></blockquote></div><blockquote type="cite"><div dir="ltr">
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<div class="">On 6 Jan 2023, at 14:39, Arnaud Masson <<a href="mailto:arnaud.masson@fr.ibm.com" class="">arnaud.masson@fr.ibm.com</a>> wrote:</div>
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<span lang="EN-US" class="">Ron,<o:p class=""></o:p></span></div>
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<span lang="EN-US" class="">My concern is that Loom (in its current form) greatly improves scalability on IO-bound requests and but introduce a new fairness problem regarding CPU-bound request.<o:p class=""></o:p></span></div>
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<div>Why are you concerned about a problem no one has reported yet? As I said, we are very interested in fixing problems, but we don’t know how to fix non-problems. If someone shows us a problem — a server that misbehaves under some loads — we’ll try to address
it. Even if there is a problem with scheduling, and even if time-sharing could solve it, we still don’t know what kind of time-sharing algorithm to employ until we see the actual problem, so there’s nothing we can do about it until we know what it actually
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<div>It’s not that I’m resistant to adding time-sharing. Far from it. But until we have a problem-reproducer that we can test and mark as fixed, we really can’t tell if the time-sharing that we were to introduce today is the time-sharing that would solve the
problem we are yet to see.</div>
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<span lang="EN-US" class="">Of course generally on web apps, most requests are IO-bound (http, jdbc) but I do have seen CPU-bound requests on prod (sometimes accidental).</span></div>
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Have you encountered a problem with virtual threads in servers with occasional CPU-bound requests?</div>
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<span lang="EN-US" class="">I don’t use Loom on prod today (I guess not a lot of people do since it’s still preview), so if you are asking if I see a production issue, answer is no.</span></div>
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Have you seen a problem with a server misbehaving in testing, then?</div>
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<div>You need to understand that I’m not teasing you. I actually want people to report problems so that we could make the product better by fixing them. But merely saying that there could be a problem and that some sketch of a solution could address it doesn’t
give us anything actionable that would help us improve the product.</div>
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<span lang="EN-US" class="">I suppose if I want to migrate to loom and be safe, I can increase the number of native carriers in the underlying pool (N >> core count).<o:p class=""></o:p></span></div>
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<span lang="EN-US" class="">It’s just that if there was timesharing in Loom, I don’t see why vthreads would not be systematically used (almost blindly, for CPU-bound and IO-bound).</span></div>
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<div>But servers based on thread pools have *worse* fairness than virtual threads: they don’t share as well even when not at 100% CPU. I don’t understand why you think more workers would make you safer when you’re not sure whether or not time-sharing helps
server workloads at all.</div>
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<span lang="EN-US" class="">I’m curious, when do you think</span><span lang="EN-US" class=""><span class="Apple-converted-space"> </span>preemptive time sharing (as implemented in various OSes for decades) is useful?</span></div>
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<div>Time-sharing in *non* realtime kernels is crucial useful to keep a system responsive to operator interaction in the presence of a few background tasks that can saturate the CPU; without it, the operator isn’t even able to terminate resource-hungry processes
(as would happen in early Windows versions). But transaction processing in servers is different. You have tens of thousands of tasks, if they consume a significant amount CPU then you’re overcommitted by orders of magnitude — indeed, OS time sharing is not
able to make servers well-behaved at 100% CPU — but responsiveness to operator intervention is still preserved thanks to the OS time-sharing.</div>
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