Change Details

Now that a difference of 1 in priority level is significant, we can reduce the span of kernel threads. Only four distinct levels are necessary for the bottom half (3 base levels and arguably an additional one for demoted interrupt threads that run for full time slices so that they finally don't compete with other ones). To leave room for other possible uses, we settle on 8 levels. Given the symbolic constants for the top half, 10 levels are currently necessary. We settle on 16 levels. This allows to enlarge the timesharing range, which covers ULE's both interactive and batch range, to 168 distinct levels, from less than 64 ones for ULE (as of before the changes to make it use a single runqueue and have 256 distinct levels per runqueue) and 34 ones for 4BSD. While here, note that the realtime range is required to have at least 32 priority levels since: - POSIX mandates at least 32 distinct levels for the SCHED_RR/SCHED_FIFO scheduling policies. - We directly map contiguous priority levels ('sched_priority') of these scheduling policies to distinct, contiguous internal priority levels. On the other hand, having at least 32 priority levels is enough to guarantee compliance to the POSIX requirement mentioned above because different internal priority levels are treated differently since commit "runq: Switch to 256 levels". While here, list explicit change restrictions for the realtime and idle range.

Please see overview of project at D45393. Now that a difference of 1 in priority level is significant, we can reduce the span of kernel threads. Only four distinct levels are necessary for the bottom half (3 base levels and arguably an additional one for demoted interrupt threads that run for full time slices so that they finally don't compete with other ones). To leave room for other possible uses, we settle on 8 levels. Given the symbolic constants for the top half, 10 levels are currently necessary. We settle on 16 levels. This allows to enlarge the timesharing range, which covers ULE's both interactive and batch range, to 168 distinct levels, from less than 64 ones for ULE (as of before the changes to make it use a single runqueue and have 256 distinct levels per runqueue) and 34 ones for 4BSD. While here, note that the realtime range is required to have at least 32 priority levels since: - POSIX mandates at least 32 distinct levels for the SCHED_RR/SCHED_FIFO scheduling policies. - We directly map contiguous priority levels ('sched_priority') of these scheduling policies to distinct, contiguous internal priority levels. On the other hand, having at least 32 priority levels is enough to guarantee compliance to the POSIX requirement mentioned above because different internal priority levels are treated differently since commit "runq: Switch to 256 levels". While here, list explicit change restrictions for the realtime and idle range.

Please see overview of project at D45393. Now that a difference of 1 in priority level is significant, we can reduce the span of kernel threads. Only four distinct levels are necessary for the bottom half (3 base levels and arguably an additional one for demoted interrupt threads that run for full time slices so that they finally don't compete with other ones). To leave room for other possible uses, we settle on 8 levels. Given the symbolic constants for the top half, 10 levels are currently necessary. We settle on 16 levels. This allows to enlarge the timesharing range, which covers ULE's both interactive and batch range, to 168 distinct levels, from less than 64 ones for ULE (as of before the changes to make it use a single runqueue and have 256 distinct levels per runqueue) and 34 ones for 4BSD. While here, note that the realtime range is required to have at least 32 priority levels since: - POSIX mandates at least 32 distinct levels for the SCHED_RR/SCHED_FIFO scheduling policies. - We directly map contiguous priority levels ('sched_priority') of these scheduling policies to distinct, contiguous internal priority levels. On the other hand, having at least 32 priority levels is enough to guarantee compliance to the POSIX requirement mentioned above because different internal priority levels are treated differently since commit "runq: Switch to 256 levels". While here, list explicit change restrictions for the realtime and idle range.