Re: [PATCH -next] mm/hugetlb_cgroup: introduce peak and rsvd.peak to v2

[Michal Hocko]

On Tue 09-07-24 20:47:30, xiujianfeng wrote:
> 
> 
> On 2024/7/9 0:04, Michal Hocko wrote:
> > On Mon 08-07-24 21:40:39, xiujianfeng wrote:
> >>
> >>
> >> On 2024/7/8 20:48, Michal Hocko wrote:
> >>> On Wed 03-07-24 13:38:04, Andrew Morton wrote:
> >>>> On Wed, 3 Jul 2024 10:45:56 +0800 xiujianfeng <xiujianfeng@xxxxxxxxxx> wrote:
> >>>>
> >>>>>
> >>>>>
> >>>>> On 2024/7/3 9:58, Andrew Morton wrote:
> >>>>>> On Tue, 2 Jul 2024 12:57:28 +0000 Xiu Jianfeng <xiujianfeng@xxxxxxxxxx> wrote:
> >>>>>>
> >>>>>>> Introduce peak and rsvd.peak to v2 to show the historical maximum
> >>>>>>> usage of resources, as in some scenarios it is necessary to configure
> >>>>>>> the value of max/rsvd.max based on the peak usage of resources.
> >>>>>>
> >>>>>> "in some scenarios it is necessary" is not a strong statement.  It
> >>>>>> would be helpful to fully describe these scenarios so that others can
> >>>>>> better understand the value of this change.
> >>>>>>
> >>>>>
> >>>>> Hi Andrew,
> >>>>>
> >>>>> Is the following description acceptable for you?
> >>>>>
> >>>>>
> >>>>> Since HugeTLB doesn't support page reclaim, enforcing the limit at
> >>>>> page fault time implies that, the application will get SIGBUS signal
> >>>>> if it tries to fault in HugeTLB pages beyond its limit. Therefore the
> >>>>> application needs to know exactly how many HugeTLB pages it uses before
> >>>>> hand, and the sysadmin needs to make sure that there are enough
> >>>>> available on the machine for all the users to avoid processes getting
> >>>>> SIGBUS.
> >>>
> >>> yes, this is pretty much a definition of hugetlb.
> >>>
> >>>>> When running some open-source software, it may not be possible to know
> >>>>> the exact amount of hugetlb it consumes, so cannot correctly configure
> >>>>> the max value. If there is a peak metric, we can run the open-source
> >>>>> software first and then configure the max based on the peak value.
> >>>
> >>> I would push back on this. Hugetlb workloads pretty much require to know
> >>> the number of hugetlb pages ahead of time. Because you need to
> >>> preallocate them for the global hugetlb pool. What I am really missing
> >>> in the above justification is an explanation of how come you know how to
> >>> configure the global pool but you do not know that for a particular
> >>> cgroup. How exactly do you configure the global pool then?
> >>
> >> Yes, in this scenario, it's indeed challenging to determine the
> >> appropriate size for the global pool. Therefore, a feasible approach is
> >> to initially configure a larger value. Once the software is running
> >> within the container successfully, the maximum value for the container
> >> and the size of the system's global pool can be determined based on the
> >> peak value, otherwise, increase the size of the global pool and try
> >> again. so I believe the peak metric is useful for this scenario.
> > 
> > This sounds really backwards to me. Not that I care much about peak
> > value itself. It is not really anything disruptive to add nor maintain
> > but this approach to configuring the system just feels completely wrong.
> > You shouldn't be really using hugetlb cgroup controller if you do not
> > have a very specific idea about expected and therefore allowed hugetlb
> > pool consumption.
> > 
> 
> Thanks for sharing your thoughts.
> 
> Since the peak metric exists in the legacy hugetlb controller, do you
> have any idea what scenario it's used for? I found it was introduced by
> commit abb8206cb077 ("hugetlb/cgroup: add hugetlb cgroup control
> files"), however there is no any description about the scenario.

I do not remember but I suspect this is mimicts other cgroupv1
interfaces.

-- 
Michal Hocko
SUSE Labs