<HTML><BODY style="word-wrap: break-word; -khtml-nbsp-mode: space; -khtml-line-break: after-white-space; "><BR><DIV><DIV>On Oct 11, 2007, at 17:45, Brian Aker wrote:</DIV><BR class="Apple-interchange-newline"><BLOCKQUOTE type="cite"><SPAN class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 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; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><DIV>Not even close to being atomic. Not only do you have issues with multicore, you have worse issues with SMP because of the costs around synchronizing the variables across the CPU transport (aka the bus between the CPU's).</DIV></SPAN></BLOCKQUOTE><DIV><BR class="khtml-block-placeholder"></DIV><DIV><SPAN class="Apple-tab-span" style="white-space:pre"> </SPAN>Short of pinning all of the increments to a single CPU, you're just going to have to deal with synchronizing this state.</DIV><BR><BLOCKQUOTE type="cite"><SPAN class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 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; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><DIV>When incrementing/changing values you need to wrap the write in a mutex if you want to be sure of the change.</DIV></SPAN></BLOCKQUOTE><BR></DIV><DIV><SPAN class="Apple-tab-span" style="white-space:pre"> </SPAN>You don't need a mutex if you have CAS. Java's AtomicInteger is implemented using a volatile integer (volatile mostly means that it can't be stored in a CPU cache, but also is used to establish a happens-before relationship with other threads on reads and writes).</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV><SPAN class="Apple-tab-span" style="white-space:pre"> </SPAN>So, given a facility for cache line synchronization and a CAS, I imagine you'll end up with a lot of code that looks like this (from Java's AtomicInteger):</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV> public final int addAndGet(int delta) {</DIV><DIV> for (;;) {</DIV><DIV> int current = get();</DIV><DIV> int next = current + delta;</DIV><DIV> if (compareAndSet(current, next))</DIV><DIV> return next;</DIV><DIV> }</DIV><DIV> }</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV><SPAN class="Apple-tab-span" style="white-space:pre"> </SPAN>glib has something similar as well. It's not guaranteed to be lock-free, but it can be done on a few platforms anyway.</DIV><BR><DIV> <SPAN class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 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; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><DIV>-- </DIV><DIV>Dustin Sallings</DIV><BR class="Apple-interchange-newline"></SPAN> </DIV><BR></BODY></HTML>