<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0; "><span class="Apple-style-span" style="border-collapse: separate; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; -webkit-text-decorations-in-effect: none; 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text-indent: 0px; -webkit-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><span class="Apple-style-span" style="border-collapse: separate; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; -webkit-text-decorations-in-effect: none; text-indent: 0px; -webkit-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><span class="Apple-style-span" style="border-collapse: separate; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; -webkit-text-decorations-in-effect: none; text-indent: 0px; -webkit-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><div><font class="Apple-style-span" color="#0000FF" face="'Gill Sans'"><span class="Apple-style-span" style="font-size: medium;">Yes, good point. It really is a question of how much does the safe-point check cost. Perhaps for a "systems" language like Modula-3 it is important that we not tax code that doesn't use threads or locks. But in today's world code like that is less common, no? And Modula-3 positively encourages them. On the other hand, Modula-3 demands programmers explicitly declare their locks (unlike Java which says any object can be a lock or CV), so perhaps retaining the referential transparency of explicit locks (you don't pay unless you use them) is more in the Modula-3 "systems" programming way of thinking.</span></font></div><div><font class="Apple-style-span" color="#0000FF" face="'Gill Sans'"><span class="Apple-style-span" style="font-size: medium;"><br></span></font></div><div><font class="Apple-style-span" color="#0000FF" face="'Gill Sans'"><span class="Apple-style-span" style="font-size: medium;">There are other ways to do the safe-points. One could "poison" (access-protect) a page that the thread simply reads a location from at the safe-points, whereupon it will trap.</span></font></div><div><font class="Apple-style-span" color="#0000FF" face="'Gill Sans'"><span class="Apple-style-span" style="font-size: medium;"><br></span></font></div><div><font class="Apple-style-span" color="#0000FF" face="'Gill Sans'"><span class="Apple-style-span" style="font-size: medium;">But in some experiments we have done with Java, it is more important that the thread simply ack, rather than having actually to take a trap and come to a screeching halt in a signal handler. And, since most threads are usually blocked or executing some external system call, you don't even have to wait for the ack.</span></font></div><div><font class="Apple-style-span" color="#0000FF" face="'Gill Sans'"><span class="Apple-style-span" style="font-size: medium;"><br></span></font></div><div><span class="Apple-style-span" style="font-size: medium; ">On 15 Dec 2009, at 09:08, <a href="mailto:hendrik@topoi.pooq.com">hendrik@topoi.pooq.com</a> wrote:</span></div></span></span></span></span></span></span></span></span></div></span></span></div><div><br class="Apple-interchange-newline"><blockquote type="cite"><div>On Mon, Dec 14, 2009 at 11:39:08PM -0500, Tony Hosking wrote:<br><blockquote type="cite">So, the basic scenario is as follows. Briefly... We should ditch <br></blockquote><blockquote type="cite">signals as a mechanism for stopping threads. The compiler should <br></blockquote><blockquote type="cite">inject a "safe-point" at each procedure call and backward branch. At <br></blockquote><blockquote type="cite">safe-points, the thread polls a thread-local to see if the run-time <br></blockquote><blockquote type="cite">system needs it to step out-of-line for some reason.<br></blockquote><br>...<br>...<br><blockquote type="cite"><br></blockquote><blockquote type="cite">The point is that uncontended locks should cost almost nothing.<br></blockquote><br>Nothing? All those safe-points cost time, don't they, even when no <br>locks are being used at all?<br><br>Is this analogous to charging everyone for the overhead involved in <br>recursion, even when no recursion is being used?<br><br>-- hendrik<br></div></blockquote></div><br></body></html>