Date: Mon, 21 Apr 2008 16:10:42 -0400 (EDT) From: Andrew Gallatin <gallatin@cs.duke.edu> To: Jeff Roberson <jroberson@jroberson.net> Cc: arch@freebsd.org Subject: Re: monitor/mwait support for idle Message-ID: <18444.62681.319881.638165@grasshopper.cs.duke.edu> In-Reply-To: <20080419004911.R942@desktop> References: <20080419004911.R942@desktop>
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Jeff Roberson writes: > http://people.freebsd.org/~jeff/mwait.diff > > This patch implements support for the x86/amd64 monitor and mwait > instructions in the idle loop. This also implements idle loop selection > via a sysctl string. The following loops are supported, in > decreasing order of performance and power consumption: > > spin - Simply returns > mwait - Always use mwait to sleep. CPU enters C0 or C1 depending on > how busy it is. > mwait_hlt - Use mwait when busy but fall back to hlt/acpi when not. > hlt - pure hlt loop > acpi - uses acpi_cpu_idle if available and hlt if not. This is the > default. > Something which may be a bit confusing is that machines like recent Core2 Xeons will go into C1E when hlt is executed, depending on an MSR setting that most BIOSes enable (bit 25, MSR 0x1a0). I think C1E might be a deeper sleep than what is reached by mwait. I confess that I don't know much about C1E, other than it kills network io intensive performance, and I turn off this (mis)feature whenever I can. It will be interesting to see how much mwait helps. I'm sure it will save power over my current workaround of disabling hlt altogether :) Drew
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