Date: Thu, 30 Mar 2000 08:46:57 +0200 From: Poul-Henning Kamp <phk@critter.freebsd.dk> To: Mike Smith <msmith@FreeBSD.ORG> Cc: Alfred Perlstein <bright@wintelcom.net>, Matthew Dillon <dillon@apollo.backplane.com>, freebsd-current@FreeBSD.ORG Subject: Re: Using packed structs to gain cheap SMP primatives Message-ID: <2921.954398817@critter.freebsd.dk> In-Reply-To: Your message of "Wed, 29 Mar 2000 22:31:17 -0800." <200003300631.WAA00497@mass.cdrom.com>
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In message <200003300631.WAA00497@mass.cdrom.com>, Mike Smith writes: >> Yesterday I was looking at how Linux handles the gettimeofday stuff >> without locking thier sys_tz variable, well it seems they don't care >> or I'm missing something important. >> >> They just don't lock it, not that settimeofday will be called all that >> often but it leaves me wondering what we can do about this, effectively >> we can pack our tz (sys_tz in Linux) into a 32bit value which should >> afford us read/write atomicity on every platform I'm aware of. >> >> In fact this can be quite effective for certain types of data structures, >> even though our 'struct timezone' is two ints we can pack it into two >> uint16 and pack a private structure, then copy it to a stack and expand >> it into the user's address space. > >It would be cheaper just to lock the bloody thing, although you can't >pack all the significance of a timeval into 16 bits anyway (in a fashion >that's going to make many people happy). Or do the "stable-storage" thing with it, and just grab a copy of the pointer (which will be an atomic op). See timecounters for an example. -- Poul-Henning Kamp FreeBSD coreteam member phk@FreeBSD.ORG "Real hackers run -current on their laptop." FreeBSD -- It will take a long time before progress goes too far! To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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