On Tue 16-07-24 08:47:59, David Finkel wrote:
On Tue, Jul 16, 2024 at 3:20 AM Michal Hocko mhocko@suse.com wrote:
On Mon 15-07-24 16:46:36, David Finkel wrote:
On Mon, Jul 15, 2024 at 4:38 PM David Finkel davidf@vimeo.com wrote:
Other mechanisms for querying the peak memory usage of either a process or v1 memory cgroup allow for resetting the high watermark. Restore parity with those mechanisms.
For example:
- Any write to memory.max_usage_in_bytes in a cgroup v1 mount resets the high watermark.
- writing "5" to the clear_refs pseudo-file in a processes's proc directory resets the peak RSS.
This change copies the cgroup v1 behavior so any write to the memory.peak and memory.swap.peak pseudo-files reset the high watermark to the current usage.
This behavior is particularly useful for work scheduling systems that need to track memory usage of worker processes/cgroups per-work-item. Since memory can't be squeezed like CPU can (the OOM-killer has opinions),
I do not understand the OOM-killer reference here. Why does it matter? Could you explain please?
Sure, we're attempting to bin-packing work based on past items of the same type. With CPU, we can provision for the mean CPU-time per-wall-time to get a lose "cores" concept that we use for binpacking. With CPU, if we end up with a bit of contention, everything just gets a bit slower while the schedule arbitrates among cgroups.
However, with memory, you only have so much physical memory for the outer memcg. If we pack things too tightly on memory, the OOM-killer is going to kill something to free up memory. In some cases that's fine, but provisioning for the peak memory for that "type" of work-item mostly avoids this issue.
It is still not clear to me how the memory reclaim falls into that. Are your workloads mostly unreclaimable (e.g. anon mostly consumers without any swap)? Why I am asking? Well, if the workload's memory is reclaimable then the peak memory consumption is largely misleading because an unknown portion of that memory consumption is hidden by the reclaimed portion of it. This is not really specific to the write handlers to reset the value though so I do not want to digress this patch too much. I do not have objections to the patch itself. Clarifying the usecase with your followup here would be nice.
Thanks for the clarification!