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How are cpu.cfs_ period_us defined in cgroups and kernel.sched_cfs_bandwidth_slice_us related?

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1

I've been trying to understand how the Completely Fair Scheduler in the Linux kernel does CPU bandwidth control.

After many readings, I still cannot wrap my head around how the CFS allows for each cgroup to define its own CFS period and how that ties in to the kernel.sched_cfs_bandwidth_slice_us sysctl setting.

Example A:
On a single CPU machine and there are two cgroups with different CFS periods defined. The quotas of each cgroup represent 50% of CPU time, totalling 100%.

cgroup-A: cpu.cfs_period_us=100ms
 cpu.cfs_quota_us=50ms
cgroup-B: cpu.cfs_period_us=1000ms
 cpu.cfs_quota_us=500ms

If each cgroup has a single process running, how are these processes scheduled given the CFS period (assuming both processes constantly require CPU time)?

Example B: What happens when cgroup-B have a higher quota than what's actually physically possible to fulfill, ie. cgroup-B set a 90% of the CPU time, and cgroup-A continues to set a quota to be 50% of the CPU time.

cgroup-A: cpu.cfs_period_us=100ms
 cpu.cfs_quota_us=50ms
cgroup-B: cpu.cfs_period_us=1000ms
 cpu.cfs_quota_us=900ms

How would CFS schedule the processes in this case when the quota is oversubscribed?

Thanks!

linux-kernel cgroups

share|improve this question

asked 24 mins ago

Otto YiuOtto Yiu

61

New contributor

Otto Yiu is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I think by typing this question out... I'm starting to gain some clarity. Correct me if I'm wrong here. If cfs_period and quota is merely used for accounting, where the period is a timer which periodically reset the usage in CPU time per cgroup - and where quota is used to compare with actual usage. Then only if the usage exceeds quota during the evaluation period, do the process get throttled and pre-empted. Does that sound sane?
  
  – Otto Yiu
  11 mins ago
  

  
  
  
  

add a comment | 

1

I've been trying to understand how the Completely Fair Scheduler in the Linux kernel does CPU bandwidth control.

After many readings, I still cannot wrap my head around how the CFS allows for each cgroup to define its own CFS period and how that ties in to the kernel.sched_cfs_bandwidth_slice_us sysctl setting.

Example A:
On a single CPU machine and there are two cgroups with different CFS periods defined. The quotas of each cgroup represent 50% of CPU time, totalling 100%.

cgroup-A: cpu.cfs_period_us=100ms
 cpu.cfs_quota_us=50ms
cgroup-B: cpu.cfs_period_us=1000ms
 cpu.cfs_quota_us=500ms

If each cgroup has a single process running, how are these processes scheduled given the CFS period (assuming both processes constantly require CPU time)?

Example B: What happens when cgroup-B have a higher quota than what's actually physically possible to fulfill, ie. cgroup-B set a 90% of the CPU time, and cgroup-A continues to set a quota to be 50% of the CPU time.

cgroup-A: cpu.cfs_period_us=100ms
 cpu.cfs_quota_us=50ms
cgroup-B: cpu.cfs_period_us=1000ms
 cpu.cfs_quota_us=900ms

How would CFS schedule the processes in this case when the quota is oversubscribed?

Thanks!

linux-kernel cgroups

share|improve this question

asked 24 mins ago

Otto YiuOtto Yiu

61

New contributor

Otto Yiu is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I think by typing this question out... I'm starting to gain some clarity. Correct me if I'm wrong here. If cfs_period and quota is merely used for accounting, where the period is a timer which periodically reset the usage in CPU time per cgroup - and where quota is used to compare with actual usage. Then only if the usage exceeds quota during the evaluation period, do the process get throttled and pre-empted. Does that sound sane?
  
  – Otto Yiu
  11 mins ago
  

  
  
  
  

add a comment | 

I've been trying to understand how the Completely Fair Scheduler in the Linux kernel does CPU bandwidth control.

After many readings, I still cannot wrap my head around how the CFS allows for each cgroup to define its own CFS period and how that ties in to the kernel.sched_cfs_bandwidth_slice_us sysctl setting.

Example A:
On a single CPU machine and there are two cgroups with different CFS periods defined. The quotas of each cgroup represent 50% of CPU time, totalling 100%.

cgroup-A: cpu.cfs_period_us=100ms
 cpu.cfs_quota_us=50ms
cgroup-B: cpu.cfs_period_us=1000ms
 cpu.cfs_quota_us=500ms

If each cgroup has a single process running, how are these processes scheduled given the CFS period (assuming both processes constantly require CPU time)?

Example B: What happens when cgroup-B have a higher quota than what's actually physically possible to fulfill, ie. cgroup-B set a 90% of the CPU time, and cgroup-A continues to set a quota to be 50% of the CPU time.

cgroup-A: cpu.cfs_period_us=100ms
 cpu.cfs_quota_us=50ms
cgroup-B: cpu.cfs_period_us=1000ms
 cpu.cfs_quota_us=900ms

How would CFS schedule the processes in this case when the quota is oversubscribed?

Thanks!

linux-kernel cgroups

share|improve this question

asked 24 mins ago

Otto YiuOtto Yiu

61

New contributor

Otto Yiu is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

cgroup-A: cpu.cfs_period_us=100ms
 cpu.cfs_quota_us=50ms
cgroup-B: cpu.cfs_period_us=1000ms
 cpu.cfs_quota_us=500ms

cgroup-A: cpu.cfs_period_us=100ms
 cpu.cfs_quota_us=50ms
cgroup-B: cpu.cfs_period_us=1000ms
 cpu.cfs_quota_us=900ms

share|improve this question

asked 24 mins ago

Otto YiuOtto Yiu

61

New contributor

Otto Yiu is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share|improve this question

asked 24 mins ago

Otto YiuOtto Yiu

61

New contributor

Otto Yiu is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked 24 mins ago

Otto YiuOtto Yiu

61

New contributor

Otto Yiu is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked 24 mins ago

Otto YiuOtto Yiu

61