<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Sep 2, 2017, at 3:19 PM, Pierre Habouzit via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div style="font-family: Helvetica; font-size: 12px; 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;" class=""><blockquote type="cite" class=""><div class="">On Sep 2, 2017, at 11:15 AM, Chris Lattner <<a href="mailto:clattner@nondot.org" class="">clattner@nondot.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><span class="" style="font-family: SFMono-Light; 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; float: none; display: inline !important;">On Aug 31, 2017, at 7:24 PM, Pierre Habouzit <</span><a href="mailto:pierre@habouzit.net" class="" style="font-family: SFMono-Light; 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;">pierre@habouzit.net</a><span class="" style="font-family: SFMono-Light; 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; float: none; display: inline !important;">> wrote:</span><br class="" style="font-family: SFMono-Light; 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;"><div class="" style="font-family: SFMono-Light; 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;"><blockquote type="cite" class=""><br class="Apple-interchange-newline"><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space;">I fail at Finding the initial mail and am quite late to the party of commenters, but there are parts I don't undertsand or have questions about.<br class=""><br class=""><blockquote class="" style="margin: 0px 0px 0px 40px; border: none; padding: 0px;"><h3 class="" style="box-sizing: border-box; margin-top: 24px; margin-bottom: 16px; font-size: 1.25em; line-height: 1.25; color: rgb(36, 41, 46); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; background-color: rgb(255, 255, 255);">Scalable Runtime</h3><p class="" style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);">[...]</p><p class="" style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);">The one problem I anticipate with GCD is that it doesn't scale well enough: server developers in particular will want to instantiate hundreds of thousands of actors in their application, at least one for every incoming network connection. The programming model is substantially harmed when you have to be afraid of creating too many actors: you have to start aggregating logically distinct stuff together to reduce # queues, which leads to complexity and loses some of the advantages of data isolation.</p></blockquote><div class=""><br class=""></div>What do you mean by this?</div></div></blockquote><div class=""><br class=""></div><div class="">My understanding is that GCD doesn’t currently scale to 1M concurrent queues / tasks.</div></div></div></blockquote><div class=""><br class=""></div><div class="">It completely does provided these 1M queues / tasks are organized on several well known independent contexts.</div><div class="">The place where GCD "fails" at is that if you target your individual serial queues to the global concurrent queues (a.k.a. root queues) which means "please pool, do your job", then yes it doesn't scale, because we take these individual serial queues as proxies for OS threads.</div><div class=""><br class=""></div><div class="">If however you target these queues to either:</div><div class="">- new serial queues to segregate your actors per subsystem yourself</div><div class="">- or some more constrained pool than what the current GCD runtime offers (where we don't create threads to run your work nearly as eagerly)</div><div class=""><br class=""></div><div class="">Then I don't see why the current implementation of GCD wouldn't scale.</div></div></div></blockquote><div><br class=""></div></div>More importantly, the basic interface of GCD doesn't seem to prevent an implementation from scaling to fill the resource constraints of a machine. The interface to dispatch queues does not imply any substantial persistent state besides the task queue itself, and tasks have pretty minimal quiescent storage requirements. Queue-hopping is an unfortunate overhead, but a constant-time overhead doesn't really damage scalability and can be addressed without a major overhaul of the basic runtime interface. OS threads can be blocked by tasks, but that's not a Dispatch-specific problem, and any solution that would fix it in other runtimes would equally fix it in Dispatch.<div class=""><br class=""></div><div class="">Now, an arbitrarily-scaling concurrent system is probably a system that's destined to eventually become distributed, and there's a strong argument that unbounded queues are an architectural mistake in a distributed system: instead, every channel of communication should have an opportunity to refuse further work, and the entire system should be architected to handle such failures gracefully. But I think that can be implemented reasonably on top of a runtime where most <i class="">local</i> queues are still unbounded and "optimistic".</div><div class=""><br class=""></div><div class=""><div class=""><div class=""><div class="">John.</div></div></div></div></body></html>