On Sun, Dec 06, 2020 at 12:25:45AM +0100, Thomas Gleixner wrote: > 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. One good client for the task isolation is Open Data Plane. There are even some code stubs supposed to enable isolation where needed. > > 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... My experiments say that typical delay caused by dry IPI or syscall is 2000-20000 'ticks'. Typical delay caused by cache miss is 3-30 ticks. To guarantee cache / memory bandwidth, one can use resctrl. Linux has implementation of it for x86 only, but arm64 has support for for resctrl on CPU side. Thanks, Yury > Thanks, > > tglx
