lkml.org 
[lkml]   [2022]   [Feb]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v7 12/12] mm: multigenerational LRU: documentation
    On Mon, Feb 21, 2022 at 06:47:25PM -0700, Yu Zhao wrote:
    > On Mon, Feb 21, 2022 at 2:02 AM Mike Rapoport <rppt@kernel.org> wrote:
    > >
    > > On Tue, Feb 15, 2022 at 08:22:10PM -0700, Yu Zhao wrote:
    > > > > Please consider splitting "enable" and "features" attributes.
    > > >
    > > > How about s/Features/Components/?
    > >
    > > I meant to use two attributes:
    > >
    > > /sys/kernel/mm/lru_gen/enable for the main breaker, and
    > > /sys/kernel/mm/lru_gen/features (or components) for the branch breakers
    >
    > It's a bit superfluous for my taste. I generally consider multiple
    > items to fall into the same category if they can be expressed by a
    > type of array, and I usually pack an array into a single file.
    >
    > From your last review, I gauged this would be too overloaded for your
    > taste. So I'd be happy to make the change if you think two files look
    > more intuitive from user's perspective.

    I do think that two attributes are more user-friendly, but I don't feel
    strongly about it.

    > > > > As for the descriptions, what is the user-visible effect of these features?
    > > > > How different modes of clearing the access bit are reflected in, say, GUI
    > > > > responsiveness, database TPS, or probability of OOM?
    > > >
    > > > These remain to be seen :) I just added these switches in v7, per Mel's
    > > > request from the meeting we had. These were never tested in the field.
    > >
    > > I see :)
    > >
    > > It would be nice to have a description or/and examples of user-visible
    > > effects when there will be some insight on what these features do.
    >
    > How does the following sound?
    >
    > Clearing the accessed bit in large batches can theoretically cause
    > lock contention (mmap_lock), and if it happens the 0x0002 switch can
    > disable this feature. In this case the multigenerational LRU suffers a
    > minor performance degradation.
    > Clearing the accessed bit in non-leaf page table entries was only
    > verified on Intel and AMD, and if it causes problems on other x86
    > varieties the 0x0004 switch can disable this feature. In this case the
    > multigenerational LRU suffers a negligible performance degradation.

    LGTM

    > > > > > +:Debugfs interface: ``/sys/kernel/debug/lru_gen`` has the following
    > > > >
    > > > > Is debugfs interface relevant only for datacenters?
    > > >
    > > > For the moment, yes.
    > >
    > > And what will happen if somebody uses these interfaces outside
    > > datacenters? As soon as there is a sysfs intefrace, somebody will surely
    > > play with it.
    > >
    > > I think the job schedulers might be the most important user of that
    > > interface, but the documentation should not presume it is the only user.
    >
    > Other ideas are more like brainstorming than concrete use cases, e.g.,
    > for desktop users, these interface can in theory speed up hibernation
    > (suspend to disk); for VM users, they can again in theory support auto
    > ballooning. These niches are really minor and less explored compared
    > with the data center use cases which have been dominant.
    >
    > I was hoping we could focus on the essential and take one step at a
    > time. Later on, if there is additional demand and resource, then we
    > expand to cover more use cases.

    Apparently I was not clear :)

    I didn't mean that you should describe other use-cases, I rather suggested
    to make the documentation more neutral, e.g. using "a user writes to this
    file ..." instead of "job scheduler writes to a file ...". Or maybe add a
    sentence in the beginning of the "Data centers" section, for instance:

    Data centers
    ------------

    + A representative example of multigenerational LRU users are job
    schedulers.

    Data centers want to optimize job scheduling (bin packing) to improve
    memory utilizations. Job schedulers need to estimate whether a server


    --
    Sincerely yours,
    Mike.

    \
     
     \ /
      Last update: 2022-02-23 11:59    [W:2.323 / U:0.020 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site