"Rafael J. Wysocki" <raf...@kernel.org> writes: > On Sat, Sep 15, 2018 at 8:53 AM Francisco Jerez <curroje...@riseup.net> wrote: >> >> "Rafael J. Wysocki" <r...@rjwysocki.net> writes: >> >> > On Tuesday, September 11, 2018 7:35:15 PM CEST Francisco Jerez wrote: >> >> >> >> "Rafael J. Wysocki" <r...@rjwysocki.net> writes: >> >> >> >> > On Thursday, September 6, 2018 6:20:08 AM CEST Francisco Jerez wrote: >> >> > >> >> >> Srinivas Pandruvada <srinivas.pandruv...@linux.intel.com> writes: >> >> >>=20 >> >> >> > [...] >> >> >> > >> >> >> >> > >=3D20 >> >> >> >> > > This patch causes a number of statistically significant >> >> >> >> > > regressions >> >> >> >> > > (with significance of 1%) on the two systems I've tested it >> >> >> >> > > on. On >> >> >> >> > > my >> >> >> >> >=3D20 >> >> >> >> > Sure. These patches are targeted to Atom clients where some of >> >> >> >> > these >> >> >> >> > server like workload may have some minor regression on few watts >> >> >> >> > TDP >> >> >> >> > parts. >> >> >> >>=3D20 >> >> >> >> Neither the 36% regression of fs-mark, the 21% regression of sqlite, >> >> >> >> nor >> >> >> >> the 10% regression of warsaw qualify as small. And most of the test >> >> >> >> cases on the list of regressions aren't exclusively server-like, if >> >> >> >> at >> >> >> >> all. Warsaw, gtkperf, jxrendermark and lightsmark are all graphics >> >> >> >> benchmarks -- Latency is as important if not more for interactive >> >> >> >> workloads than it is for server workloads. In the case of a >> >> >> >> conflict >> >> >> >> like the one we're dealing with right now between optimizing for >> >> >> >> throughput (e.g. for the maximum number of requests per second) and >> >> >> >> optimizing for latency (e.g. for the minimum request duration), you >> >> >> >> are >> >> >> >> more likely to be concerned about the former than about the latter >> >> >> >> in >> >> >> >> a >> >> >> >> server setup. >> >> >> > >> >> >> > Eero, >> >> >> > Please add your test results here. >> >> >> > >> >> >> > No matter which algorithm you do, there will be variations. So you >> >> >> > have >> >> >> > to look at the platforms which you are targeting. For this >> >> >> > platform=3D= >> >> 20 >> >> >> > number one item is use of less turbo and hope you know why? >> >> >>=20 >> >> >> Unfortunately the current controller uses turbo frequently on Atoms for >> >> >> TDP-limited graphics workloads regardless of IOWAIT boosting. IOWAIT >> >> >> boosting simply exacerbated the pre-existing energy efficiency problem. >> >> > >> >> > My current understanding of the issue at hand is that using IOWAIT >> >> > boosti= >> >> ng >> >> > on Atoms is a regression relative to the previous behavior. >> >> >> >> Not universally. IOWAIT boosting helps under roughly the same >> >> conditions on Atom as it does on big core, so applying this patch will >> >> necessarily cause regressions too (see my reply from Sep. 3 for some >> >> numbers), and won't completely restore the previous behavior since it >> >> simply decreases the degree of IOWAIT boosting applied without being >> >> able to avoid it (c.f. the series I'm working on that does something >> >> similar to IOWAIT boosting when it's able to determine it's actually >> >> CPU-bound, which prevents energy inefficient behavior for non-CPU-bound >> >> workloads that don't benefit from a higher CPU clock frequency anyway). >> > >> > Well, OK. That doesn't seem to be a clear-cut regression situation, then, >> > since getting back is not desirable, apparently. >> > >> > Or would it restore the previous behavior if we didn't do any IOWAIT >> > boosting on Atoms at all? >> > >> >> > That is what Srinivas is trying to address here AFAICS. >> >> > >> >> > Now, you seem to be saying that the overall behavior is suboptimal and >> >> > the >> >> > IOWAIT boosting doesn't matter that much, >> >> >> >> I was just saying that IOWAIT boosting is less than half of the energy >> >> efficiency problem, and this patch only partially addresses that half of >> >> the problem. >> > >> > Well, fair enough, but there are two things to consider here, the general >> > energy-efficiency problem and the difference made by IOWAIT boosting. >> > >> > If the general energy-efficiency problem had existed for a relatively long >> > time, but it has got worse recently due to the IOWAIT boosting, it still >> > may be desirable to get the IOWAIT boosting out of the picture first >> > and then get to the general problem. >> > >> >> IMHO what is needed in order to address the IOWAIT boosting energy >> efficiency problem is roughly the same we need in order to address the >> other energy efficiency problem: A mechanism along the lines of [1] >> allowing us to determine whether the workload is IO-bound or not. In >> the former case IOWAIT boosting won't be able to improve the performance >> of the workload since the limiting factor is the IO throughput, so it >> will only increase the energy usage, potentially exacerbating the >> bottleneck if the IO device is an integrated GPU. In the latter case >> where the CPU and IO devices being waited on are both underutilized it >> makes sense to optimize for low latency more aggressively (certainly >> more aggressively than this patch does) which will increase the >> utilization of the IO devices until at least one IO device becomes a >> bottleneck, at which point the throughput of the system becomes roughly >> independent of the CPU frequency and we're back to the former case. >> >> [1] https://patchwork.kernel.org/patch/10312259/ > > I remember your argumentation above from the previous posts and I'm > not questioning it. I don't see much point in repeating arguments > that have been given already. > > My question was whether or not there was a regression related > specifically to adding the IOWAIT boosting mechanism that needed to be > addressed separately. I gather from the discussion so far that this > is not the case. >
There possibly was some slight graphics performance regression when i915 started doing IO waits, but i915 didn't have support for any BXT+ devices back then, which are the ones most severely hurt by it, so it probably didn't cause a significant drop in most available benchmark numbers and went unnoticed. Regardless of whether there was a regression, I don't see the need of fixing the IOWAIT issue separately from the other energy efficiency issues of the active mode governor, because they both admit a common solution... > Thanks, > Rafael
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