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    SubjectRe: [Lhms-devel] [PATCH 0/7] Fragmentation Avoidance V19

    * Andy Nelson <andy@thermo.lanl.gov> wrote:

    > I think it was Martin Bligh who wrote that his customer gets 25%
    > speedups with big pages. That is peanuts compared to my factor 3.4
    > (search comp.arch for John Mashey's and my name at the University of
    > Edinburgh in Jan/Feb 2003 for a conversation that includes detailed
    > data about this), but proves the point that it is far more than just
    > me that wants big pages.

    ok, this posting of you seems to be it:

    http://groups.google.com/group/comp.sys.sgi.admin/browse_thread/thread/39884db861b7db15/e0332608c52a17e3?lnk=st&q=&rnum=35#e0332608c52a17e3

    | Timing for the tree traveral+gravity calculation were
    |
    | 16MBpages 1MBpages 64kpages
    | 1 * * 2361.8s
    | 8 86.4s 198.7s 298.1s
    | 16 43.5s 99.2s 148.9s
    | 32 22.1s 50.1s 75.0s
    | 64 11.2s 25.3s 37.9s
    | 96 7.5s 17.1s 25.4s
    |
    | (*) test not done.
    |
    | As near as I can tell the numbers show perfect
    | linear speedup for the runs for each page size.
    |
    | Across different page sizes there is degradation
    | as follows:
    |
    | 16m --> 64k decreases by a factor 3.39 in speed
    | 16m --> 1m decreases by a factor 2.25 in speed
    | 1m --> 64k decreases by a factor 1.49 in speed

    [...]
    |
    | Sum over cpus of TLB miss times for each test:
    |
    | 16MBpages 1MBpages 64kpages
    | 1 3489s
    | 8 64.3s 1539s 3237s
    | 16 64.5s 1540s 3241s
    | 32 64.5s 1542s 3244s
    | 64 64.9s 1545s 3246s
    | 96 64.7s 1545s 3251s
    |
    | Thus the 16MB pages rarely produced page misses,
    | while the 64kB pages used up 2.5x more time than
    | the floating point operations that we wanted to
    | have. I have at least some feeling that the 16MB pages
    | rarely caused misses because with a 128 entry
    | TLB (on the R12000 cpu) that gives about 1GB of
    | addressible memory before paging is required at all,
    | which I think is quite comparable to the size of
    | the memory actually used.

    to me it seems that this slowdown is due to some inefficiency in the
    R12000's TLB-miss handling - possibly very (very!) long TLB-miss
    latencies? On modern CPUs (x86/x64) the TLB-miss latency is rarely
    visible. Would it be possible to run some benchmarks of hugetlbs vs. 4K
    pages on x86/x64?

    if my assumption is correct, then hugeTLBs are more of a workaround for
    bad TLB-miss properties of the CPUs you are using, not something that
    will inevitably happen in the future. Hence i think the 'factor 3x'
    slowdown should not be realistic anymore - or are you still running
    R12000 CPUs?

    Ingo
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