| Message ID | 20220915164411.2496380-1-kajetan.puchalski@arm.com |
|---|---|
| Headers | show |
| Series | cpuidle: teo: Introduce optional util-awareness | expand |
On Thu, Sep 15, 2022 at 9:45 AM Kajetan Puchalski <kajetan.puchalski@arm.com> wrote: > > Hi, Hi, I tried it. > > At the moment, all the available idle governors operate mainly based on their own past performance > without taking into account any scheduling information. Especially on interactive systems, this > results in them frequently selecting a deeper idle state and then waking up before its target > residency is hit, thus leading to increased wakeup latency and lower performance with no power > saving. For 'menu' while web browsing on Android for instance, those types of wakeups ('too deep') > account for over 24% of all wakeups. > > At the same time, on some platforms C0 can be power efficient enough to warrant wanting to prefer > it over C1. Sleeps that happened in C0 while they could have used C1 ('too shallow') only save > less power than they otherwise could have. Too deep sleeps, on the other hand, harm performance > and nullify the potential power saving from using C1 in the first place. While taking this into > account, it is clear that on balance it is preferable for an idle governor to have more too shallow > sleeps instead of more too deep sleeps. > > Currently the best available governor under this metric is TEO which on average results in less than > half the percentage of too deep sleeps compared to 'menu', getting much better wakeup latencies and > increased performance in the process. > > This proposed optional extension to TEO would specifically tune it for minimising too deep > sleeps and minimising latency to achieve better performance. To this end, before selecting the next > idle state it uses the avg_util signal of a CPU's runqueue in order to determine to what extent the > CPU is being utilized. This util value is then compared to a threshold defined as a percentage of > the cpu's capacity (capacity >> 6 ie. ~1.5% in the current implementation). That seems quite a bit too low to me. However on my processor the energy cost of using idle state 0 verses anything deeper is very high, so I do not have a good way to test. Processor: Intel(R) Core(TM) i5-10600K CPU @ 4.10GHz On an idle system : with only Idle state 0 enabled, processor package power is ~46 watts. with only idle state 1 enabled, processor package power is ~2.6 watts with all idle states enabled, processor package power is ~1.4 watts > If the util is above the > threshold, the governor directly selects the shallowest available idle state. If the util is below > the threshold, the governor defaults to the TEO metrics mechanism to try to select the deepest > available idle state based on the closest timer event and its own past correctness. > > Effectively this functions like a governor that on the fly disables deeper idle states when there > are things happening on the cpu and then immediately reenables them as soon as the cpu isn't > being utilized anymore. > > Initially I am sending this as a patch for TEO to visualize the proposed mechanism and simplify > the review process. An alternative way of implementing it while not interfering > with existing TEO code would be to fork TEO into a separate but mostly identical for the time being > governor (working name 'idleutil') and then implement util-awareness there, so that the two > approaches can coexist and both be available at runtime instead of relying on a compile-time option. > I am happy to send a patchset doing that if you think it's a cleaner approach than doing it this way. I would prefer the two to coexist for testing, as it makes it easier to manually compare some areas of focus. > > This approach can outperform all the other currently available governors, at least on mobile device > workloads, which is why I think it is worth keeping as an option. > > Additionally, in my view, the reason why it makes more sense to implement this type of mechanism > inside a governor rather than outside using something like QoS or some other way to disable certain > idle states on the fly are the governor's metrics. If we were disabling idle states and reenabling > them without the governor 'knowing' about it, the governor's metrics would end up being updated > based on state selections not caused by the governor itself. This could interfere with the > correctness of said metrics as that's not what they were designed for as far as I understand. > This approach skips metrics updates whenever a state was selected based on the util and not based > on the metrics. > > There is no particular attachment or reliance on TEO for this mechanism, I simply chose to base > it on TEO because it performs the best out of all the available options and I didn't think there was > any point in reinventing the wheel on the side of computing governor metrics. If a > better approach comes along at some point, there's no reason why the same idle aware mechanism > couldn't be used with any other metrics algorithm. That would, however, require implemeting it as > a separate governor rather than a TEO add-on. > > As for how the extension performs in practice, below I'll add some benchmark results I got while > testing this patchset. > > Pixel 6 (Android 12, mainline kernel 5.18): > > 1. Geekbench 5 (latency-sensitive, heavy load test) > > The values below are gmean values across 3 back to back iteration of Geekbench 5. > As GB5 is a heavy benchmark, after more than 3 iterations intense throttling kicks in on mobile devices > resulting in skewed benchmark scores, which makes it difficult to collect reliable results. The actual > values for all of the governors can change between runs as the benchmark might be affected by factors > other than just latency. Nevertheless, on the runs I've seen, util-aware TEO frequently achieved better > scores than all the other governors. > > 'shallow' is a trivial governor that only ever selects the shallowest available state, included here > for reference and to establish the lower bound of latency possible to achieve through cpuidle. > > 'gmean too deep %' and 'gmean too shallow %' are percentages of too deep and too shallow sleeps > computed using the new trace event - cpu_idle_miss. The percentage is obtained by counting the two > types of misses over the course of a run and then dividing them by the total number of wakeups. > > | metric | menu | teo | shallow | teo + util-aware | > | ------------------------------------- | ------------- | --------------- | --------------- | --------------- | > | gmean score | 2716.4 (0.0%) | 2795 (+2.89%) | 2780.5 (+2.36%) | 2830.8 (+4.21%) | > | gmean too deep % | 16.64% | 9.61% | 0% | 4.19% | > | gmean too shallow % | 2.66% | 5.54% | 31.47% | 15.3% | > | gmean task wakeup latency (gb5) | 82.05μs (0.0%) | 73.97μs (-9.85%) | 42.05μs (-48.76%) | 66.91μs (-18.45%) | > | gmean task wakeup latency (asynctask) | 75.66μs (0.0%) | 56.58μs (-25.22%) | 65.78μs (-13.06%) | 55.35μs (-26.84%) | > > In case of this benchmark, the difference in latency does seem to translate into better scores. > > Additionally, here's a set of runs of Geekbench done after holding the phone in > the fridge for exactly an hour each time in order to minimise the impact of thermal issues. > > | metric | menu | teo | teo + util-aware | > | ------------------------------------- | ------------- | --------------- | --------------- | > | gmean multicore score | 2792.1 (0.0%) | 2845.2 (+1.9%) | 2857.4 (+2.34%) | > | gmean single-core score | 1048.3 (0.0%) | 1052.6 (+0.41%) | 1055.3 (+0.67%) | > > 2. PCMark Web Browsing (non latency-sensitive, normal usage test) > > The table below contains gmean values across 20 back to back iterations of PCMark 2 Web Browsing. > > | metric | menu | teo | shallow | teo + util-aware | > | ------------------------- | ------------- | --------------- | --------------- | --------------- | > | gmean score | 6283.0 (0.0%) | 6262.9 (-0.32%) | 6258.4 (-0.39%) | 6323.7 (+0.65%) | > | gmean too deep % | 24.15% | 10.32% | 0% | 3.2% | > | gmean too shallow % | 2.81% | 7.68% | 27.69% | 17.189% | > | gmean power usage [mW] | 209.1 (0.0%) | 187.8 (-10.17%) | 205.5 (-1.71%) | 205 (-1.96%) | > | gmean task wakeup latency | 204.6μs (0.0%) | 184.39μs (-9.87%) | 95.55μs (-53.3%) | 95.98μs (-53.09%) | > > As this is a web browsing benchmark, the task for which the wakeup latency was recorded was Chrome's > rendering task, ie CrRendererMain. The latency improvement for the actual benchmark task was very > similar. > > In this case the large latency improvement does not translate into a notable increase in benchmark score as > this particular benchmark mainly responds to changes in operating frequency. Nevertheless, the small power > saving compared to menu with no decrease in benchmark score indicate that there are no regressions for this > type of workload while using this governor. > > Note: The results above were as mentioned obtained on the 5.18 kernel. Results for Geekbench obtained after > backporting CFS patches from the most recent mainline can be found in the pdf linked below [1]. > The results and improvements still hold up but the numbers change slightly. Additionally, the pdf contains > plots for all the relevant results obtained with this and other idle governors. > > At the very least this approach seems promising so I wanted to discuss it in RFC form first. > Thank you for taking your time to read this! There might be a way forward for my type of processor if the algorithm were to just reduce the idle depth by 1 instead of all the way to idle state 0. Not sure. It seems to bypass all that the teo governor is attempting to achieve. For a single periodic workflow at any work sleep frequency (well, I test 5 hertz to 411 hertz) and very light workload: Processor package powers for 73 hertz work/sleep frequency: teo: ~1.5 watts menu: ~1.5 watts util: ~19 watts For 12 periodic workflow threads at 73 hertz work/sleep frequency (well, I test 5 hertz to 411 hertz) and very workload: Processor package powers: teo: ~2.8watts menu: ~2.8 watts util: ~49 watts My test computer is a server, with no gui. I started a desktop linux VM guest that isn't doing much: teo: ~1.8 watts menu: ~1.8 watts util: ~7.8 watts > > -- > Kajetan > > [1] https://github.com/mrkajetanp/lisa-notebooks/blob/a2361a5b647629bfbfc676b942c8e6498fb9bd03/idle_util_aware.pdf > > > Kajetan Puchalski (1): > cpuidle: teo: Introduce optional util-awareness > > drivers/cpuidle/Kconfig | 12 +++++ > drivers/cpuidle/governors/teo.c | 86 +++++++++++++++++++++++++++++++++ > 2 files changed, 98 insertions(+) > > -- > 2.37.1 >
Hi Rafael, Just a gentle ping here. Could you please take a look at this discussion? I'd like to address some comments I received, especially on the subject of making it reduce the state by one as opposed to going all the way to 0 to account for different hardware and how we can accomodate different architectures in the implementation of that mechanism. Before I send a v2 it'd be great to know your opinions on this and whether I should still send it as a TEO patch or fork it into a separate governor and make the changes there as both Doug and I seem to prefer. Thank you in advance for you time, Kajetan On Thu, Sep 15, 2022 at 05:44:10PM +0100, Kajetan Puchalski wrote: > At the very least this approach seems promising so I wanted to discuss it in RFC form first. > Thank you for taking your time to read this! > > [1] https://github.com/mrkajetanp/lisa-notebooks/blob/a2361a5b647629bfbfc676b942c8e6498fb9bd03/idle_util_aware.pdf > > Kajetan Puchalski (1): > cpuidle: teo: Introduce optional util-awareness > > drivers/cpuidle/Kconfig | 12 +++++ > drivers/cpuidle/governors/teo.c | 86 +++++++++++++++++++++++++++++++++ > 2 files changed, 98 insertions(+) > > -- > 2.37.1 >
Hi, At the moment, all the available idle governors operate mainly based on their own past performance without taking into account any scheduling information. Especially on interactive systems, this results in them frequently selecting a deeper idle state and then waking up before its target residency is hit, thus leading to increased wakeup latency and lower performance with no power saving. For 'menu' while web browsing on Android for instance, those types of wakeups ('too deep') account for over 24% of all wakeups. At the same time, on some platforms C0 can be power efficient enough to warrant wanting to prefer it over C1. Sleeps that happened in C0 while they could have used C1 ('too shallow') only save less power than they otherwise could have. Too deep sleeps, on the other hand, harm performance and nullify the potential power saving from using C1 in the first place. While taking this into account, it is clear that on balance it is preferable for an idle governor to have more too shallow sleeps instead of more too deep sleeps. Currently the best available governor under this metric is TEO which on average results in less than half the percentage of too deep sleeps compared to 'menu', getting much better wakeup latencies and increased performance in the process. This proposed optional extension to TEO would specifically tune it for minimising too deep sleeps and minimising latency to achieve better performance. To this end, before selecting the next idle state it uses the avg_util signal of a CPU's runqueue in order to determine to what extent the CPU is being utilized. This util value is then compared to a threshold defined as a percentage of the cpu's capacity (capacity >> 6 ie. ~1.5% in the current implementation). If the util is above the threshold, the governor directly selects the shallowest available idle state. If the util is below the threshold, the governor defaults to the TEO metrics mechanism to try to select the deepest available idle state based on the closest timer event and its own past correctness. Effectively this functions like a governor that on the fly disables deeper idle states when there are things happening on the cpu and then immediately reenables them as soon as the cpu isn't being utilized anymore. Initially I am sending this as a patch for TEO to visualize the proposed mechanism and simplify the review process. An alternative way of implementing it while not interfering with existing TEO code would be to fork TEO into a separate but mostly identical for the time being governor (working name 'idleutil') and then implement util-awareness there, so that the two approaches can coexist and both be available at runtime instead of relying on a compile-time option. I am happy to send a patchset doing that if you think it's a cleaner approach than doing it this way. This approach can outperform all the other currently available governors, at least on mobile device workloads, which is why I think it is worth keeping as an option. Additionally, in my view, the reason why it makes more sense to implement this type of mechanism inside a governor rather than outside using something like QoS or some other way to disable certain idle states on the fly are the governor's metrics. If we were disabling idle states and reenabling them without the governor 'knowing' about it, the governor's metrics would end up being updated based on state selections not caused by the governor itself. This could interfere with the correctness of said metrics as that's not what they were designed for as far as I understand. This approach skips metrics updates whenever a state was selected based on the util and not based on the metrics. There is no particular attachment or reliance on TEO for this mechanism, I simply chose to base it on TEO because it performs the best out of all the available options and I didn't think there was any point in reinventing the wheel on the side of computing governor metrics. If a better approach comes along at some point, there's no reason why the same idle aware mechanism couldn't be used with any other metrics algorithm. That would, however, require implemeting it as a separate governor rather than a TEO add-on. As for how the extension performs in practice, below I'll add some benchmark results I got while testing this patchset. Pixel 6 (Android 12, mainline kernel 5.18): 1. Geekbench 5 (latency-sensitive, heavy load test) The values below are gmean values across 3 back to back iteration of Geekbench 5. As GB5 is a heavy benchmark, after more than 3 iterations intense throttling kicks in on mobile devices resulting in skewed benchmark scores, which makes it difficult to collect reliable results. The actual values for all of the governors can change between runs as the benchmark might be affected by factors other than just latency. Nevertheless, on the runs I've seen, util-aware TEO frequently achieved better scores than all the other governors. 'shallow' is a trivial governor that only ever selects the shallowest available state, included here for reference and to establish the lower bound of latency possible to achieve through cpuidle. 'gmean too deep %' and 'gmean too shallow %' are percentages of too deep and too shallow sleeps computed using the new trace event - cpu_idle_miss. The percentage is obtained by counting the two types of misses over the course of a run and then dividing them by the total number of wakeups. | metric | menu | teo | shallow | teo + util-aware | | ------------------------------------- | ------------- | --------------- | --------------- | --------------- | | gmean score | 2716.4 (0.0%) | 2795 (+2.89%) | 2780.5 (+2.36%) | 2830.8 (+4.21%) | | gmean too deep % | 16.64% | 9.61% | 0% | 4.19% | | gmean too shallow % | 2.66% | 5.54% | 31.47% | 15.3% | | gmean task wakeup latency (gb5) | 82.05μs (0.0%) | 73.97μs (-9.85%) | 42.05μs (-48.76%) | 66.91μs (-18.45%) | | gmean task wakeup latency (asynctask) | 75.66μs (0.0%) | 56.58μs (-25.22%) | 65.78μs (-13.06%) | 55.35μs (-26.84%) | In case of this benchmark, the difference in latency does seem to translate into better scores. Additionally, here's a set of runs of Geekbench done after holding the phone in the fridge for exactly an hour each time in order to minimise the impact of thermal issues. | metric | menu | teo | teo + util-aware | | ------------------------------------- | ------------- | --------------- | --------------- | | gmean multicore score | 2792.1 (0.0%) | 2845.2 (+1.9%) | 2857.4 (+2.34%) | | gmean single-core score | 1048.3 (0.0%) | 1052.6 (+0.41%) | 1055.3 (+0.67%) | 2. PCMark Web Browsing (non latency-sensitive, normal usage test) The table below contains gmean values across 20 back to back iterations of PCMark 2 Web Browsing. | metric | menu | teo | shallow | teo + util-aware | | ------------------------- | ------------- | --------------- | --------------- | --------------- | | gmean score | 6283.0 (0.0%) | 6262.9 (-0.32%) | 6258.4 (-0.39%) | 6323.7 (+0.65%) | | gmean too deep % | 24.15% | 10.32% | 0% | 3.2% | | gmean too shallow % | 2.81% | 7.68% | 27.69% | 17.189% | | gmean power usage [mW] | 209.1 (0.0%) | 187.8 (-10.17%) | 205.5 (-1.71%) | 205 (-1.96%) | | gmean task wakeup latency | 204.6μs (0.0%) | 184.39μs (-9.87%) | 95.55μs (-53.3%) | 95.98μs (-53.09%) | As this is a web browsing benchmark, the task for which the wakeup latency was recorded was Chrome's rendering task, ie CrRendererMain. The latency improvement for the actual benchmark task was very similar. In this case the large latency improvement does not translate into a notable increase in benchmark score as this particular benchmark mainly responds to changes in operating frequency. Nevertheless, the small power saving compared to menu with no decrease in benchmark score indicate that there are no regressions for this type of workload while using this governor. Note: The results above were as mentioned obtained on the 5.18 kernel. Results for Geekbench obtained after backporting CFS patches from the most recent mainline can be found in the pdf linked below [1]. The results and improvements still hold up but the numbers change slightly. Additionally, the pdf contains plots for all the relevant results obtained with this and other idle governors. At the very least this approach seems promising so I wanted to discuss it in RFC form first. Thank you for taking your time to read this! -- Kajetan [1] https://github.com/mrkajetanp/lisa-notebooks/blob/a2361a5b647629bfbfc676b942c8e6498fb9bd03/idle_util_aware.pdf Kajetan Puchalski (1): cpuidle: teo: Introduce optional util-awareness drivers/cpuidle/Kconfig | 12 +++++ drivers/cpuidle/governors/teo.c | 86 +++++++++++++++++++++++++++++++++ 2 files changed, 98 insertions(+)