Pavel, On Sat, Dec 05 2020 at 21:40, Pavel Machek wrote: > So... what kind of guarantees does this aim to provide / what tasks it > is useful for? > > For real time response, we have other approaches. Depends on your requirements. Some problems are actually better solved with busy polling. See below. > If you want to guarantee performnace of the "isolated" task... I don't > see how that works. Other tasks on the system still compete for DRAM > bandwidth, caches, etc... Applications which want to run as undisturbed as possible. There is quite a range of those: - Hardware in the loop simulation is today often done with that crude approach of "offlining" a CPU and then instead of playing dead jumping to a preloaded bare metal executable. That's a horrible hack and impossible to debug, but gives them the results they need to achieve. These applications are well optimized vs. cache and memory foot print, so they don't worry about these things too much and they surely don't run on SMI and BIOS value add inflicted machines. Don't even think about waiting for an interrupt to achieve what these folks are doing. So no, there are problems which a general purpose realtime OS cannot solve ever. - HPC computations on large data sets. While the memory foot print is large the access patterns are cache optimized. The problem there is that any unnecessary IPI, tick interrupt or whatever nuisance is disturbing the carefully optimized cache usage and alone getting rid of the timer interrupt gained them measurable performance. Even very low single digit percentage of runtime saving is valuable for these folks because the compute time on such beasts is expensive. - Realtime guests in KVM. With posted interrupts and a fully populated host side page table there is no point in running host side interrupts or IPIs for random accounting or whatever purposes as they affect the latency in the guest. With all the side effects mitigated and a properly set up guest and host it is possible to get to a zero exit situation after the bootup phase which means pretty much matching bare metal behaviour. Yes, you can do that with e.g. Jailhouse as well, but you lose lots of the fancy things KVM provides. And people care about these not just because they are fancy. They care because their application scenario needs them. There are more reasons why people want to be able to get as much isolation from the OS as possible but at the same time have a sane execution environment, debugging, performance monitoring and the OS provided protection mechanisms instead of horrible hacks. Isolation makes sense for a range of applications and there is no reason why Linux should not support them. Thanks, tglx