From: Roman Gushchin <[email protected]> Signed-off-by: Roman Gushchin <[email protected]> --- README | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 56 insertions(+)
diff --git a/README b/README index a24ec89..c8af02d 100644 --- a/README +++ b/README @@ -1,3 +1,59 @@ + SMART (Simultaneous Multithreading-Aware Real-Time) + +This commit series adds SMT awareness to Linux RT scheduler. + +SMART is aimed at realtime CPU-bound applications which run for a +short period of time (tens of milliseconds) and expect no interruption +(basically anything that is facing user and should finish execution as +quickly as possible). + +On our memory-hot search clusters with thousands of machines we see +10-15% more RPS compared to CFS. Most profit is gained when machine +load is around 50%. + +While we aim for full CPU utilization with SMART, some care is still +taken about CFS tasks, because it's crucial to be able to ssh on a fully +loaded machine. + +Linux realtime scheduler uses cpupri structure to distribute tasks +among CPU cores which doesn't take topology into account and thus can +make very poor scheduling decisions. SMART scheduler completely abandons +cpupri mechanism and tries to balance RT load between NUMA nodes and +doesn't place (if possible) tasks on adjacent SMT threads of a single core. + +Among other features are: +- SMART pull: if all SMT threads of a single core execute RT tasks we + migrate them whenever free core becomes available +- CFS throttling: if some core has RT tasks running, we don't enqueue + CFS tasks on the other SMT threads of this core +- SMART gathering: we migrate running CFS tasks from all SMT threads + of a core that don't have RT tasks, to the thread that has + +Usage: + +SMART is just an improvement on classic Linux realtime scheduler, thus +all tasks that have SCHED_FIFO or SCHED_RR policy use SMART. From the +command line it's possible to manipulate scheduler's policy using chrt +command. + +Also, we provide cpu.smart knob to the cpu cgroup which sets scheduler +policy of all tasks in the cgroup to SCHED_RR. + + +When not to use SMART (you still can use it, but don't expect any profit): +- target application is not bursty and its execution takes second(s) +- CPU utilization of target machine approaches 100% +- target application is true real-time application and you do care + about latency measured in microseconds + + +Copyright (c) 2013-2014, Yandex LLC + +Authors: Roman Gushchin ([email protected]), + Stanislav Fomichev ([email protected]) + +-------------------------------------------------------------------------------- + Linux kernel release 3.x <http://kernel.org/> These are the release notes for Linux version 3. Read them carefully, -- 1.9.3 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
