1. 25 2月, 2010 2 次提交
    • I
      x86, mm: Allow highmem user page tables to be disabled at boot time · 14315592
      Ian Campbell 提交于
      Distros generally (I looked at Debian, RHEL5 and SLES11) seem to
      enable CONFIG_HIGHPTE for any x86 configuration which has highmem
      enabled. This means that the overhead applies even to machines which
      have a fairly modest amount of high memory and which therefore do not
      really benefit from allocating PTEs in high memory but still pay the
      price of the additional mapping operations.
      
      Running kernbench on a 4G box I found that with CONFIG_HIGHPTE=y but
      no actual highptes being allocated there was a reduction in system
      time used from 59.737s to 55.9s.
      
      With CONFIG_HIGHPTE=y and highmem PTEs being allocated:
        Average Optimal load -j 4 Run (std deviation):
        Elapsed Time 175.396 (0.238914)
        User Time 515.983 (5.85019)
        System Time 59.737 (1.26727)
        Percent CPU 263.8 (71.6796)
        Context Switches 39989.7 (4672.64)
        Sleeps 42617.7 (246.307)
      
      With CONFIG_HIGHPTE=y but with no highmem PTEs being allocated:
        Average Optimal load -j 4 Run (std deviation):
        Elapsed Time 174.278 (0.831968)
        User Time 515.659 (6.07012)
        System Time 55.9 (1.07799)
        Percent CPU 263.8 (71.266)
        Context Switches 39929.6 (4485.13)
        Sleeps 42583.7 (373.039)
      
      This patch allows the user to control the allocation of PTEs in
      highmem from the command line ("userpte=nohigh") but retains the
      status-quo as the default.
      
      It is possible that some simple heuristic could be developed which
      allows auto-tuning of this option however I don't have a sufficiently
      large machine available to me to perform any particularly meaningful
      experiments. We could probably handwave up an argument for a threshold
      at 16G of total RAM.
      
      Assuming 768M of lowmem we have 196608 potential lowmem PTE
      pages. Each page can map 2M of RAM in a PAE-enabled configuration,
      meaning a maximum of 384G of RAM could potentially be mapped using
      lowmem PTEs.
      
      Even allowing generous factor of 10 to account for other required
      lowmem allocations, generous slop to account for page sharing (which
      reduces the total amount of RAM mappable by a given number of PT
      pages) and other innacuracies in the estimations it would seem that
      even a 32G machine would not have a particularly pressing need for
      highmem PTEs. I think 32G could be considered to be at the upper bound
      of what might be sensible on a 32 bit machine (although I think in
      practice 64G is still supported).
      
      It's seems questionable if HIGHPTE is even a win for any amount of RAM
      you would sensibly run a 32 bit kernel on rather than going 64 bit.
      Signed-off-by: NIan Campbell <ian.campbell@citrix.com>
      LKML-Reference: <1266403090-20162-1-git-send-email-ian.campbell@citrix.com>
      Signed-off-by: NH. Peter Anvin <hpa@zytor.com>
      14315592
    • T
      x86: Do not reserve brk for DMI if it's not going to be used · e808bae2
      Thadeu Lima de Souza Cascardo 提交于
      This will save 64K bytes from memory when loading linux if DMI is
      disabled, which is good for embedded systems.
      Signed-off-by: NThadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
      LKML-Reference: <1265758732-19320-1-git-send-email-cascardo@holoscopio.com>
      Signed-off-by: NH. Peter Anvin <hpa@zytor.com>
      e808bae2
  2. 18 2月, 2010 2 次提交
  3. 17 2月, 2010 36 次提交