Date: Mon, 21 Apr 2008 12:43:42 -1000 (HST) From: Jeff Roberson <jroberson@jroberson.net> To: Andrew Gallatin <gallatin@cs.duke.edu> Cc: arch@freebsd.org Subject: Re: monitor/mwait support for idle Message-ID: <20080421124258.M942@desktop> In-Reply-To: <18444.62681.319881.638165@grasshopper.cs.duke.edu> References: <20080419004911.R942@desktop> <18444.62681.319881.638165@grasshopper.cs.duke.edu>
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On Mon, 21 Apr 2008, Andrew Gallatin wrote: > > 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 :) I don't have any useful measurements for power. For performance I see as much as 10-15% in some workloads. This is due to both latency and cpu time reduction in wakeup heavy workloads that also idle a lot. Jeff > > Drew >
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