Johannes Weiner hannes@cmpxchg.org writes:
Hi Stefan,
On Fri, Feb 10, 2023 at 01:50:04PM -0800, Stefan Roesch wrote:
So far KSM can only be enabled by calling madvise for memory regions. What is required to enable KSM for more workloads is to enable / disable it at the process / cgroup level.
Use case: The madvise call is not available in the programming language. An example for this are programs with forked workloads using a garbage collected language without pointers. In such a language madvise cannot be made available.
In addition the addresses of objects get moved around as they are garbage collected. KSM sharing needs to be enabled "from the outside" for these type of workloads.
It would be good to expand on the argument that Rik made about the interpreter being used for things were there are no merging opportunities, and the KSM scanning overhead isn't amortized.
There is a fundamental mismatch in scopes. madvise() is a workload-local decision, whereas sizable sharing opportunities may or may not exist across multiple workloads. Only a higher-level entity like a job scheduler can know for certain whether it's running one or more instances of a job. That job scheduler in turn doesn't have the necessary knowledge of the workload's internals to make targeted and well-timed advise calls with, say, process_madvise().
This also applies to the security concerns brought up in previous threads. An individual workload doesn't know what else is running on the machine, so it needs to be highly conservative about what it can give up for system-wide merging. However, if the system is dedicated to running multiple jobs within the same security domain, it's the job scheduler that knows that sharing isn't a problem, and even desirable.
So I think this series makes sense, but it would be good to expand a bit on the reasoning and address the security aspect in the cover/doc.
These are good points Johannes, I'll elaborate on them with the next version of the patch.
Stefan Roesch (19): mm: add new flag to enable ksm per process mm: add flag to __ksm_enter mm: add flag to __ksm_exit call mm: invoke madvise for all vmas in scan_get_next_rmap_item mm: support disabling of ksm for a process mm: add new prctl option to get and set ksm for a process
The implementation looks sound to me as well.
I think it would be a bit easier to review if you folded these ^^^ patches, the tools patch below, and the prctl selftests, all into one single commit. It's one logical change. This way the new flags and helper functions can be reviewed against the new users and callsites without having to jump back and forth between emails.
I'll fold them in the next version.
mm: split off pages_volatile function mm: expose general_profit metric docs: document general_profit sysfs knob mm: calculate ksm process profit metric mm: add ksm_merge_type() function mm: expose ksm process profit metric in ksm_stat mm: expose ksm merge type in ksm_stat docs: document new procfs ksm knobs
Same with the new knobs/stats and their documentation.
I'll fold them in the next version.
Logical splitting is easier to follow than geographical splitting.
Thanks!