lkml.org 
[lkml]   [2021]   [Sep]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH 0/2] tracing: Have trace_pid_list be a sparse array
    On Thu, 23 Sep 2021 23:35:47 -0400
    Steven Rostedt <rostedt@goodmis.org> wrote:

    > The pid_mask will start out with 1024 entries for the first 10 MSB bits.
    > This will cost 4K for 32 bit architectures and 8K for 64 bit. Each of
    > these will have a 1024 array to store the next 10 bits of the pid (another
    > 4 or 8K). These will hold an 512 byte bitmask (which will cover the LSB 12
    > bits or 4096 bits).

    Thinking about this more, I should adjust this split.

    Currently, this is a 10,10,12 split, but since the upper chunks are
    arrays of pointers, and the lower chunk is a bitmask, and that pids
    tend to be close together, I should make the lower split bigger.

    As a 4K page is 32768 bits (2^15), the lower split should be 15 bits,
    not 12. This will probably allocate better.

    Of course 32 - 15 is 17. So maybe to keep it simple, by having the two
    upper chunks still the same in size, I could have it be 14 bits for the
    lower (2048 bytes). And since pid_max can only be 2^30, we don't even
    need to cover the full 32 bits.

    30 - 14 = 16 = 8 * 2.

    Then I can make the upper chunks cover 8 bits (arrays of 256 pointers)
    and the lower chunks cove 14 bits. This would coincidentally make all
    chunks 2K in size (on 64 bit machines).

    Hmm, in that case, I can make the lower and upper chunks use the same
    memory and not separate them. They are unions after all. But that may
    be unfair for 32 bit machines, as the upper chunks are only going to be
    1K in size for them (256 * 4).

    -- Steve

    \
     
     \ /
      Last update: 2021-09-24 06:08    [W:6.740 / U:0.020 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site