1. 12 Jan, 2006 1 commit
    • akpm@osdl.org's avatar
      [PATCH] scheduler cache-hot-autodetect · 198e2f18
      akpm@osdl.org authored
      
      
      )
      
      From: Ingo Molnar <mingo@elte.hu>
      
      This is the latest version of the scheduler cache-hot-auto-tune patch.
      
      The first problem was that detection time scaled with O(N^2), which is
      unacceptable on larger SMP and NUMA systems. To solve this:
      
      - I've added a 'domain distance' function, which is used to cache
        measurement results. Each distance is only measured once. This means
        that e.g. on NUMA distances of 0, 1 and 2 might be measured, on HT
        distances 0 and 1, and on SMP distance 0 is measured. The code walks
        the domain tree to determine the distance, so it automatically follows
        whatever hierarchy an architecture sets up. This cuts down on the boot
        time significantly and removes the O(N^2) limit. The only assumption
        is that migration costs can be expressed as a function of domain
        distance - this covers the overwhelming majority of existing systems,
        and is a good guess even for more assymetric systems.
      
        [ People hacking systems that have assymetries that break this
          assumption (e.g. different CPU speeds) should experiment a bit with
          the cpu_distance() function. Adding a ->migration_distance factor to
          the domain structure would be one possible solution - but lets first
          see the problem systems, if they exist at all. Lets not overdesign. ]
      
      Another problem was that only a single cache-size was used for measuring
      the cost of migration, and most architectures didnt set that variable
      up. Furthermore, a single cache-size does not fit NUMA hierarchies with
      L3 caches and does not fit HT setups, where different CPUs will often
      have different 'effective cache sizes'. To solve this problem:
      
      - Instead of relying on a single cache-size provided by the platform and
        sticking to it, the code now auto-detects the 'effective migration
        cost' between two measured CPUs, via iterating through a wide range of
        cachesizes. The code searches for the maximum migration cost, which
        occurs when the working set of the test-workload falls just below the
        'effective cache size'. I.e. real-life optimized search is done for
        the maximum migration cost, between two real CPUs.
      
        This, amongst other things, has the positive effect hat if e.g. two
        CPUs share a L2/L3 cache, a different (and accurate) migration cost
        will be found than between two CPUs on the same system that dont share
        any caches.
      
      (The reliable measurement of migration costs is tricky - see the source
      for details.)
      
      Furthermore i've added various boot-time options to override/tune
      migration behavior.
      
      Firstly, there's a blanket override for autodetection:
      
      	migration_cost=1000,2000,3000
      
      will override the depth 0/1/2 values with 1msec/2msec/3msec values.
      
      Secondly, there's a global factor that can be used to increase (or
      decrease) the autodetected values:
      
      	migration_factor=120
      
      will increase the autodetected values by 20%. This option is useful to
      tune things in a workload-dependent way - e.g. if a workload is
      cache-insensitive then CPU utilization can be maximized by specifying
      migration_factor=0.
      
      I've tested the autodetection code quite extensively on x86, on 3
      P3/Xeon/2MB, and the autodetected values look pretty good:
      
      Dual Celeron (128K L2 cache):
      
       ---------------------
       migration cost matrix (max_cache_size: 131072, cpu: 467 MHz):
       ---------------------
                 [00]    [01]
       [00]:     -     1.7(1)
       [01]:   1.7(1)    -
       ---------------------
       cacheflush times [2]: 0.0 (0) 1.7 (1784008)
       ---------------------
      
      Here the slow memory subsystem dominates system performance, and even
      though caches are small, the migration cost is 1.7 msecs.
      
      Dual HT P4 (512K L2 cache):
      
       ---------------------
       migration cost matrix (max_cache_size: 524288, cpu: 2379 MHz):
       ---------------------
                 [00]    [01]    [02]    [03]
       [00]:     -     0.4(1)  0.0(0)  0.4(1)
       [01]:   0.4(1)    -     0.4(1)  0.0(0)
       [02]:   0.0(0)  0.4(1)    -     0.4(1)
       [03]:   0.4(1)  0.0(0)  0.4(1)    -
       ---------------------
       cacheflush times [2]: 0.0 (33900) 0.4 (448514)
       ---------------------
      
      Here it can be seen that there is no migration cost between two HT
      siblings (CPU#0/2 and CPU#1/3 are separate physical CPUs). A fast memory
      system makes inter-physical-CPU migration pretty cheap: 0.4 msecs.
      
      8-way P3/Xeon [2MB L2 cache]:
      
       ---------------------
       migration cost matrix (max_cache_size: 2097152, cpu: 700 MHz):
       ---------------------
                 [00]    [01]    [02]    [03]    [04]    [05]    [06]    [07]
       [00]:     -    19.2(1) 19.2(1) 19.2(1) 19.2(1) 19.2(1) 19.2(1) 19.2(1)
       [01]:  19.2(1)    -    19.2(1) 19.2(1) 19.2(1) 19.2(1) 19.2(1) 19.2(1)
       [02]:  19.2(1) 19.2(1)    -    19.2(1) 19.2(1) 19.2(1) 19.2(1) 19.2(1)
       [03]:  19.2(1) 19.2(1) 19.2(1)    -    19.2(1) 19.2(1) 19.2(1) 19.2(1)
       [04]:  19.2(1) 19.2(1) 19.2(1) 19.2(1)    -    19.2(1) 19.2(1) 19.2(1)
       [05]:  19.2(1) 19.2(1) 19.2(1) 19.2(1) 19.2(1)    -    19.2(1) 19.2(1)
       [06]:  19.2(1) 19.2(1) 19.2(1) 19.2(1) 19.2(1) 19.2(1)    -    19.2(1)
       [07]:  19.2(1) 19.2(1) 19.2(1) 19.2(1) 19.2(1) 19.2(1) 19.2(1)    -
       ---------------------
       cacheflush times [2]: 0.0 (0) 19.2 (19281756)
       ---------------------
      
      This one has huge caches and a relatively slow memory subsystem - so the
      migration cost is 19 msecs.
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      Signed-off-by: default avatarAshok Raj <ashok.raj@intel.com>
      Signed-off-by: default avatarKen Chen <kenneth.w.chen@intel.com>
      Cc: <wilder@us.ibm.com>
      Signed-off-by: default avatarJohn Hawkes <hawkes@sgi.com>
      Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
      Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
      198e2f18
  2. 07 Sep, 2005 1 commit
  3. 25 Jun, 2005 4 commits
  4. 16 Apr, 2005 1 commit
    • Linus Torvalds's avatar
      Linux-2.6.12-rc2 · 1da177e4
      Linus Torvalds authored
      Initial git repository build. I'm not bothering with the full history,
      even though we have it. We can create a separate "historical" git
      archive of that later if we want to, and in the meantime it's about
      3.2GB when imported into git - space that would just make the early
      git days unnecessarily complicated, when we don't have a lot of good
      infrastructure for it.
      
      Let it rip!
      1da177e4