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<div style="caret-color: rgb(0, 0, 0); color: rgb(0, 0, 0);">You are making the assumption that any kind of clever scheduling would be able to substantially improve server workloads at 100% CPU, but this hypothesis is yet to be demonstrated. To consider better
scheduling algorithms, we first need to see real-world cases where the current scheduling causes a problem that requires a solution. We can’t solve a problem before we know what it is.</div>
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<div style="caret-color: rgb(0, 0, 0); color: rgb(0, 0, 0);">What we’re looking for is something along the lines of: “I have a server based on virtual threads, and I’ve run into the following scheduling-related problem: …” I expect that such reports will take
time to show up, as they require more widespread use of virtual threads in production (or even in integration testing).</div>
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<div style="caret-color: rgb(0, 0, 0); color: rgb(0, 0, 0);">— Ron</div>
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<div class="">On 6 Jan 2023, at 10:36, 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" style="font-size: 11pt;" class="">I think what could be useful is to automatically add scheduling “weight” on vthread in the context of structured concurrency.<o:p class=""></o:p></span></div>
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<span lang="EN-US" style="font-size: 11pt;" class="">(A bit like CPU request/limit at pod level in Kubernetes.)<o:p class=""></o:p></span></div>
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<span lang="EN-US" style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div>
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<span lang="EN-US" style="font-size: 11pt;" class="">For example, let’s say I have 2 vthreads on a server processing concurrent http requests (mostly CPU-bound).<o:p class=""></o:p></span></div>
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<span lang="EN-US" style="font-size: 11pt;" class="">If one of them forks 2 sub tasks with a structured concurrency scope, I would like that the weights of the 2 requests remain globally constant:<o:p class=""></o:p></span></div>
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<span lang="EN-US" style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div>
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<span lang="EN-US" style="font-size: 11pt;" class="">Initially:<o:p class=""></o:p></span></div>
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<span lang="EN-US" style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div>
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<span lang="EN-US" style="font-size: 11pt;" class="">request 1: weight 1.0<o:p class=""></o:p></span></li><li class="MsoListParagraph" style="margin: 0cm; font-size: 10pt; font-family: Calibri, sans-serif;">
<span lang="EN-US" style="font-size: 11pt;" class="">request 2: weight 1.0<o:p class=""></o:p></span></li></ul>
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<span lang="EN-US" style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div>
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<span lang="EN-US" style="font-size: 11pt;" class="">after fork:<o:p class=""></o:p></span></div>
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<span lang="EN-US" style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div>
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<span lang="EN-US" style="font-size: 11pt;" class="">request 1 (blocked waiting for sub tasks)<o:p class=""></o:p></span></li><ul type="circle" style="margin-bottom: 0cm; margin-top: 0cm;" class="">
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<span lang="EN-US" style="font-size: 11pt;" class="">sub task 1: weight 0.5<o:p class=""></o:p></span></li><li class="MsoListParagraph" style="margin: 0cm; font-size: 10pt; font-family: Calibri, sans-serif;">
<span lang="EN-US" style="font-size: 11pt;" class="">sub task 2: weight 0.5<o:p class=""></o:p></span></li></ul>
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<span lang="EN-US" style="font-size: 11pt;" class="">request 2: weight 1.0<o:p class=""></o:p></span></li></ul>
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<span lang="EN-US" style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div>
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<span lang="EN-US" style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div>
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<span lang="EN-US" style="font-size: 11pt;" class="">if 3 cores available, the 3 actives tasks can run at 100% (and request1 will complete faster).<o:p class=""></o:p></span></div>
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<span lang="EN-US" style="font-size: 11pt;" class="">If 2 cores only, request 2 would be at 100% on a core, and the 2 sub tasks of request1 would get 50% of a core each, so request1 forking won’t have negative impact on request 2.<o:p class=""></o:p></span></div>
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<span lang="EN-US" style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div>
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<span lang="EN-US" style="font-size: 11pt;" class="">In other words, that would allow to optimize for multicore (inside a request handler) without breaking scheduling fairness.<o:p class=""></o:p></span></div>
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<span lang="EN-US" style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div>
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<span lang="EN-US" style="font-size: 11pt;" class="">Thanks<o:p class=""></o:p></span></div>
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<span lang="EN-US" style="font-size: 11pt;" class="">Arnaud</span><span lang="EN-US" style="font-size: 11pt;" class=""><o:p class=""></o:p></span></div>
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> On 5 Jan 2023, at 21:24, Robert Engels <<a href="mailto:rengels@ix.netcom.com" class="">rengels@ix.netcom.com</a>> wrote:<br class="">
><span class="Apple-converted-space"> </span><br class="">
> I think it would be better to be able to park/unpark on a monitor efficiently.<span class="Apple-converted-space"> </span></span><span style="font-size: 11pt;" class="">I think the overhead in handling that common case is more significant than expected.<span class="Apple-converted-space"> </span><br class="">
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We know; it’s work-in-progress, but will take some time.<br class="">
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> I like the idea of carrying over the Thread.priority in vthreads to allow finer control over the scheduling.<span class="Apple-converted-space"> </span><br class="">
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Scheduling with priorities can either be done well or fast but not both. Non-realtime kernels schedule relatively fast, but their priority implementation isn’t that good; realtime kernels do priorities well, but their scheduling is very slow (realtime always
trades off predictability for speed). Respecting priorities becomes even harder when there are lots and lots of threads, and problems such as priority inversion certainly don’t get easier. For the common cases where you have a small set of low-priority threads
doing work in the background, the easiest solution is just to use platform threads for those.<br class="">
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As in the case of time-sharing, the need for priorities is something that will need to be demonstrated in real projects in the field to justify addressing.<br class="">
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— Ron<o:p class=""></o:p></span></div>
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