Generally, ZFS does not use floating point. And further, use of floating point in the kernel is exceptionally rare. The kernel does not save floating point context automatically, which means that code that uses floating point needs to take special care to make sure any context from userland is saved and restored before it can use the registers itself. This rather onerous burden tends to exclude "easy" consumption of floating point.
That said, some operations -- and cryptographic ones in particular -- may use floating point registers and operations because for some architectures (sun4u rings a bell) this can make certain expensive operations go faster. I don't think this is the case for secure hash/message digest algorithms, but if you use ZFS encryption as found in Solaris 11 Express you might find that on certain systems these registers are used for performance reasons, either on the bulk crypto or on the keying operations. (More likely the latter, but my memory of these optimizations is still hazy.) Note that *if* this is done, it is only done where such an operation is a performance win, and not because any of the math is inherently floating point. So in this case, I would say that this optimization would be an advantage, rather than a disadvantage. Oh, and this usage only applies to Solaris, and is optional. I doubt FreeBSD has these particular enhancements -- indeed, IIRC, these optimizations are specific to certain classes of SPARC cpus and probably are not performed at all for x86 cpus. - Garrett -----Original Message----- From: zfs-discuss-boun...@opensolaris.org on behalf of Erik Trimble Sent: Wed 12/22/2010 12:08 PM To: Jerry Kemp; zfs-discuss@opensolaris.org Subject: Re: [zfs-discuss] stupid ZFS question - floating point operations On 12/22/2010 11:49 AM, Tomas Ögren wrote: > On 22 December, 2010 - Jerry Kemp sent me these 1,0K bytes: > >> I have a coworker, who's primary expertise is in another flavor of Unix. >> >> This coworker lists floating point operations as one of ZFS detriments. >> >> I's not really sure what he means specifically, or where he got this >> reference from. > Then maybe ask him first? Guilty until proven innocent isn't the regular > path... > >> In an effort to refute what I believe is an error or misunderstanding on >> his part, I have spent time on Yahoo, Google, the ZFS section of >> OpenSolaris.org, etc. I really haven't turned up much of anything that >> would prove or disprove his comments. The one thing I haven't done is >> to go through the ZFS source code, but its been years since I have done >> any serious programming. >> >> If someone from Oracle, or anyone on this mailing list could point me >> towards any documentation, or give me a definitive word, I would sure >> appreciate it. If there were floating point operations going on within >> ZFS, at this point I am uncertain as to what they would be. >> >> TIA for any comments, >> >> Jerry >> _______________________________________________ >> zfs-discuss mailing list >> zfs-discuss@opensolaris.org >> http://mail.opensolaris.org/mailman/listinfo/zfs-discuss > > /Tomas So far as my understanding of the codebase goes (and, while I've read a significant portion, I'm not really an expert here): Assuming he means that ZFS has a weakness of heavy floating-point calculation requirements (i.e using ZFS requires heavy FP usage), that's wrong. Like all normal filesystems, the "ordinary" operations are all integer, load, and store. The ordinary work of caching, block allocation, and fetching/writing is of course all integer-based. I can't imagine someone writing a filesystem which does such operations using floating point. A quick grep through the main ZFS sources doesn't find anything of type "double" or "float". I think he might be confused with what is happening on Checksums (which is still all Integer, but looks/sounds "expensive"). Yes, ZFS is considerably *more* compute intensive than other filesystems. However, it's all Integer, and one of the base assumptions of ZFS is that modern systems have lots of excess CPU cycles around, so stealing 5% for use with ZFS won't impact performance much, and the added features of ZFS more than make up for any CPU cycles lost. -- Erik Trimble Java System Support Mailstop: usca22-123 Phone: x17195 Santa Clara, CA Timezone: US/Pacific (GMT-0800) _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
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