So after a bit of irc discusssions yesterday it turns out that you can overclock intel gpus by quite a margin. Which makes some sense now that the gfx performance of intel chips isn't something to be completely ashamed of.
You need a few ingredients:
- An Sandybridge/Ivybridge Intel GPU (Intel HD 2000/3000/2500/4000 in marketing speak).
- A "enthusiast" motherboard which allows you to set gpu overclocking settings (higher frequency and voltage).
- Lastet drm-intel-nightly kernel branch from drm-intel git or at least a kernel built with this patch from Ben Widawsky. The patch should apply to pretty much any recent stable kernel. Without that patch gpu turbo support is broken.
- Preferably a desktop gpu with a big cooling rig. Overclocking on modern intel chips essentially just increases the turbo headroom into the unvalidated range (with the potential for hangs and corruptions in rendering). And if your cooling system can't keep up with the increased heat output the on-chip controller will quickly clamp your clocks down to the non-turbo frequency.
- Your favorite gpu benchmark. The reason for that is the thermal throttling when running in the turbo range: It happens behind the driver's back and the only observable effect is a slower gpu - the current frequency value in sysfs is still the same!
drm.debug=0xeappended to your kernel cmdline and check dmesg for the gpu overclocking support debug message:
[98650.411179] [drm:gen6_enable_rps], overclocking supported, adjusting frequency max from 1300MHz to 1300MHz
Yeah, my system doesn't really support overclocking :(
Or check in sysfs in
gt_*_freq_mhzfiles. You can also use those to limit the upper clock, which is useful for figuring out at which frequency your gpu is still stable at for a given voltage setting.
Limiting the lower clocks is usually not interesting, since it will only affect how much power the hardware can safe under intermediate loads. And you want to aggressively reduce heat output to better use the turbo range when needed.
If you play around with this please drop a comment with your results - I only have boring Intel developer board's around here which don't support overclocking. So no fun fore me. But freezer on #intel-gfx managed to overclock his desktop Ivybridge from 1.1GHz to 1.6GHz with only a small voltage increase, and fps in CS:S increased quite a bit due to that.
Happy overclocking and benchmarking
Update: In debugfs you can check the CAGF (current actual gt frequency) field in the
i915_cur_delayinfofile for the real frequency, including any effects due to thermal throttling.
Update 2: Latest kernels (and so also 3.10) will limit the gpu frequency at boot-up to the non-overclocked range. This should prevent crashes while booting, but it also means that you need to manually set the overclocked frequency limit.