On Thu, Mar 23, 2023 at 07:52:22AM -0300, Marcelo Tosatti wrote: > On Thu, Mar 23, 2023 at 08:51:14AM +0100, Michal Hocko wrote: > > On Wed 22-03-23 11:20:55, Marcelo Tosatti wrote: > > > On Wed, Mar 22, 2023 at 02:35:20PM +0100, Michal Hocko wrote: > > [...] > > > > > "Performance details for the kworker interruption: > > > > > > > > > > oslat 1094.456862: sys_mlock(start: 7f7ed0000b60, len: 1000) > > > > > oslat 1094.456971: workqueue_queue_work: ... function=vmstat_update ... > > > > > oslat 1094.456974: sched_switch: prev_comm=oslat ... ==> next_comm=kworker/5:1 ... > > > > > kworker 1094.456978: sched_switch: prev_comm=kworker/5:1 ==> next_comm=oslat ... > > > > > > > > > > The example above shows an additional 7us for the > > > > > > > > > > oslat -> kworker -> oslat > > > > > > > > > > switches. In the case of a virtualized CPU, and the vmstat_update > > > > > interruption in the host (of a qemu-kvm vcpu), the latency penalty > > > > > observed in the guest is higher than 50us, violating the acceptable > > > > > latency threshold for certain applications." > > > > > > > > Yes, I have seen that but it doesn't really give a wider context to > > > > understand why those numbers matter. > > > > > > OK. > > > > > > "In the case of RAN, a MAC scheduler with TTI=1ms, this causes >100us > > > interruption observed in a guest (which is above the safety > > > threshold for this application)." > > > > > > Is that OK? > > > > This might be a sufficient information for somebody familiar with the > > matter (not me). So no, not enough. We need to hear a more complete > > story. > > Michal, > > Please refer to > https://www.diva-portal.org/smash/get/diva2:541460/FULLTEXT01.pdf > > 2.3 Channel Dependent Scheduling > The purpose of scheduling is to decide which terminal will transmit data on which set > of resource blocks with what transport format to use. The objective is to assign > resources to the terminal such that the quality of service (QoS) requirement is fulfilled. > Scheduling decision is taken every 1 ms by base station (termed as eNodeB) as the > same length of Transmission Time Interval (TTI) in LTE system. > > In general: > > https://en.wikipedia.org/wiki/Real-time_computing > > Real-time computing (RTC) is the computer science term for hardware and > software systems subject to a "real-time constraint", for example from > event to system response.[1] Real-time programs must guarantee response > within specified time constraints, often referred to as "deadlines".[2] > > Real-time responses are often understood to be in the order of > milliseconds, and sometimes microseconds. A system not specified as > operating in real time cannot usually guarantee a response within any > timeframe, although typical or expected response times may be given. > Real-time processing fails if not completed within a specified deadline > relative to an event; deadlines must always be met, regardless of system > load. > > For example, for the MAC scheduler processing must occur every 1ms, > and a certain amount of computation takes place (and must finish before > the next 1ms timeframe). A > 50us latency spike as observed by cyclictest > is considered a "failure". If you need more detail, will have to ask someone else, because that is all I know.
