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<?xml version="1.0" encoding="UTF-8" standalone="yes"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>LKML: Paul Mackerras: Re: [patch 0/9] mutex subsystem, -V4</title><link href="/css/message.css" rel="stylesheet" type="text/css" /><link href="/css/wrap.css" rel="alternate stylesheet" type="text/css" title="wrap" /><link href="/css/nowrap.css" rel="stylesheet" type="text/css" title="nowrap" /><link href="/favicon.ico" rel="shortcut icon" /><script src="/js/simple-calendar.js" type="text/javascript"></script><script src="/js/styleswitcher.js" type="text/javascript"></script><link rel="alternate" type="application/rss+xml" title="lkml.org : last 100 messages" href="/rss.php" /><link rel="alternate" type="application/rss+xml" title="lkml.org : last messages by Paul Mackerras" href="/groupie.php?aid=37" /><script> var _paq = window._paq = window._paq || []; /* tracker methods like "setCustomDimension" should be called before "trackPageView" */ _paq.push(["setDoNotTrack", true]); _paq.push(["disableCookies"]); _paq.push(['trackPageView']); _paq.push(['enableLinkTracking']); (function() { var u="//m.lkml.org/"; _paq.push(['setTrackerUrl', u+'matomo.php']); _paq.push(['setSiteId', '1']); var d=document, g=d.createElement('script'), s=d.getElementsByTagName('script')[0]; g.async=true; g.src=u+'matomo.js'; s.parentNode.insertBefore(g,s); })(); </script></head><body onload="es.jasper.simpleCalendar.init();" itemscope="itemscope" itemtype="http://schema.org/BlogPosting"><table border="0" cellpadding="0" cellspacing="0"><tr><td width="180" align="center"><a href="/"><img style="border:0;width:135px;height:32px" src="/images/toprowlk.gif" alt="lkml.org" /></a></td><td width="32">聽</td><td class="nb"><div><a class="nb" href="/lkml"> [lkml]</a> 聽 <a class="nb" href="/lkml/2005"> [2005]</a> 聽 <a class="nb" href="/lkml/2005/12"> [Dec]</a> 聽 <a class="nb" href="/lkml/2005/12/22"> [22]</a> 聽 <a class="nb" href="/lkml/last100"> [last100]</a> 聽 <a href="/rss.php"><img src="/images/rss-or.gif" border="0" alt="RSS Feed" /></a></div><div>Views: <a href="#" class="nowrap" onclick="setActiveStyleSheet('wrap');return false;">[wrap]</a><a href="#" class="wrap" onclick="setActiveStyleSheet('nowrap');return false;">[no wrap]</a> 聽 <a class="nb" href="/lkml/mheaders/2005/12/22/270" onclick="this.href='/lkml/headers'+'/2005/12/22/270';">[headers]</a>聽 <a href="/lkml/bounce/2005/12/22/270">[forward]</a>聽 </div></td><td width="32">聽</td></tr><tr><td valign="top"><div class="es-jasper-simpleCalendar" baseurl="/lkml/"></div><div class="threadlist">Messages in this thread</div><ul class="threadlist"><li class="root"><a href="/lkml/2005/12/22/69">First message in thread</a></li><li><a href="/lkml/2005/12/22/88">Ingo Molnar</a><ul><li><a href="/lkml/2005/12/22/99">Andrew Morton</a><ul><li><a href="/lkml/2005/12/22/104">Arjan van de Ven</a><ul><li><a href="/lkml/2005/12/22/113">Andrew Morton</a><ul><li><a href="/lkml/2005/12/22/118">Alan Cox</a></li><li><a href="/lkml/2005/12/22/147">Thomas Gleixner</a></li><li><a href="/lkml/2005/12/22/181">Christoph Hellwig</a></li></ul></li></ul></li><li><a href="/lkml/2005/12/22/132">Nicolas Pitre</a></li><li class="origin"><a href="">Paul Mackerras</a></li></ul></li></ul></li></ul></td><td width="32" rowspan="2" class="c" valign="top"><img src="/images/icornerl.gif" width="32" height="32" alt="/" /></td><td class="c" rowspan="2" valign="top" style="padding-top: 1em"><table><tr><td><table><tr><td class="lp">Date</td><td class="rp" itemprop="datePublished">Fri, 23 Dec 2005 08:43:07 +1100</td></tr><tr><td class="lp">From</td><td class="rp" itemprop="author">Paul Mackerras <></td></tr><tr><td class="lp">Subject</td><td class="rp" itemprop="name">Re: [patch 0/9] mutex subsystem, -V4</td></tr></table></td><td></td></tr></table><pre itemprop="articleBody">Andrew Morton writes:<br /><br />> Ingo Molnar <mingo@elte.hu> wrote:<br />> > - 'struct mutex' is smaller: on x86, 'struct semaphore' is 20 bytes, <br />> > 'struct mutex' is 16 bytes. A smaller structure size means less RAM <br />> > footprint, and better CPU-cache utilization.<br />> <br />> Because of the .sleepers thing. Perhaps a revamped semaphore wouldn't need<br />> thsat field?<br /><br />Note that semaphores on 32-bit PPC are only 16 bytes already, since I<br />got rid of the sleepers field ages ago. The fast path is just<br />atomic_dec/inc, and the slow path needs an atomic op that does<br /><br /> x = max(x, 0) + inc<br /><br />atomically and returns the old value of x. That's easy to do with<br />lwarx/stwcx (just two more instructions than an atomic inc), and can<br />also be done quite straightforwardly with cmpxchg. Alpha, mips, s390<br />and sparc64 also use this scheme.<br /><br />In fact I would go so far as to say that I cannot see how it would be<br />possible to do a mutex any smaller or faster than a counting semaphore<br />on these architectures.<br /><br />Regards,<br />Paul.<br />-<br />To unsubscribe from this list: send the line "unsubscribe linux-kernel" in<br />the body of a message to majordomo@vger.kernel.org<br />More majordomo info at <a href="http://vger.kernel.org/majordomo-info.html">http://vger.kernel.org/majordomo-info.html</a><br />Please read the FAQ at <a href="http://www.tux.org/lkml/">http://www.tux.org/lkml/</a><br /><br /></pre></td><td width="32" rowspan="2" class="c" valign="top"><img src="/images/icornerr.gif" width="32" height="32" alt="\" /></td></tr><tr><td align="right" valign="bottom"> 聽 </td></tr><tr><td align="right" valign="bottom">聽</td><td class="c" valign="bottom" style="padding-bottom: 0px"><img src="/images/bcornerl.gif" width="32" height="32" alt="\" /></td><td class="c">聽</td><td class="c" valign="bottom" style="padding-bottom: 0px"><img src="/images/bcornerr.gif" width="32" height="32" alt="/" /></td></tr><tr><td align="right" valign="top" colspan="2"> 聽 </td><td class="lm">Last update: 2005-12-22 22:45 聽聽 [from the cache]<br />漏2003-2020 <a href="http://blog.jasper.es/"><span itemprop="editor">Jasper Spaans</span></a>|hosted at <a href="https://www.digitalocean.com/?refcode=9a8e99d24cf9">Digital Ocean</a> and my Meterkast|<a href="http://blog.jasper.es/categories.html#lkml-ref">Read the blog</a></td><td>聽</td></tr></table><script language="javascript" src="/js/styleswitcher.js" type="text/javascript"></script></body></html>