lkml.org 
[lkml]   [2014]   [May]   [9]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: vmstat: On demand vmstat workers V4
    On Fri, 9 May 2014, Christoph Lameter wrote:
    > On Fri, 9 May 2014, Thomas Gleixner wrote:
    > > I think we agreed long ago, that for the whole HPC FULL_NOHZ stuff you
    > > have to sacrify at least one CPU for housekeeping purposes of all
    > > kinds, timekeeping, statistics and whatever.
    >
    > Ok how do I figure out that cpu? I'd rather have a specific cpu that
    > never changes.

    I followed the full nohz development only losely, but back then when
    all started here at my place with frederic, we had a way to define the
    housekeeper cpu. I think we lazily had it hardwired to 0 :)

    That probably changed, but I'm sure there is still a way to define a
    housekeeper. And we should simply force the timekeeping on that
    housekeeper. That comes with the price, that the housekeeper is not
    allowed to go deep idle, but I bet that in HPC scenarios this does not
    matter at all simply because the whole machine is under full load.

    Frederic?

    > > So if you have a housekeeper, then it makes absolutely no sense at all
    > > to move it around in circles.
    > >
    > > Can you please enlighten me why we need this at all?
    >
    > The vmstat kworker thread checks every 2 seconds if there are vmstat
    > updates that need to be folded into the global statistics. This is not
    > necessary if the application is running and no OS services are being used.
    > Thus we could switch off vmstat updates and avoid taking the processor
    > away from the application.
    >
    > This has also been noted by multiple other people at was brought up at the
    > mm summit by others who noted the same issues.

    I understand why you want to get this done by a housekeeper, I just
    did not understand why we need this whole move it around business is
    required.

    Thanks,

    tglx



    \
     
     \ /
      Last update: 2014-05-09 17:41    [W:2.638 / U:0.020 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site