• Thomas Petazzoni's avatar
    ARM: 8180/1: mm: implement no-highmem fast path in kmap_atomic_pfn() · 9ff0bb5b
    Thomas Petazzoni authored
    Since CONFIG_HIGHMEM got enabled on ARMv5 Kirkwood, we have noticed a
    very significant drop in networking performance. The test were
    conducted on an OpenBlocks A7 board. Without this patch, the outgoing
    performance measured with iperf are:
    
     - highmem OFF, TSO OFF   544 Mbit/s
     - highmem OFF, TSO ON	  942 Mbit/s
     - highmem ON,  TSO OFF   306 Mbit/s
     - highmem ON,  TSO ON    246 Mbit/s
    
    On this Kirkwood platform, the L2 cache is a Feroceon cache, and with
    this cache, all the range operations have to be done on virtual
    addresses and not physical addresses. Therefore, whenever
    CONFIG_HIGHMEM is enabled, the cache maintenance operations call
    kmap_atomic_pfn() and kunmap_atomic().
    
    However, kmap_atomic_pfn() does not implement the same fast path for
    non-highmem pages as the one implemented in kmap_atomic(), and this is
    one of the reason for the performance drop. While this patch does not
    fully restore the performances, it clearly improves them a lot:
    
          	      	        without patch  with patch
    
     - highmem ON, TSO OFF   306 Mbit/s     387 Mbit/s
     - highmem ON, TSO ON    246 Mbit/s     434 Mbit/s
    
    We're still far from the !CONFIG_HIGHMEM performances, but it does
    improve a bit the situation.
    
    Thanks a lot to Ezequiel Garcia and Gregory Clement for all the
    testing work around this topic.
    Signed-off-by: default avatarThomas Petazzoni <thomas.petazzoni@free-electrons.com>
    Signed-off-by: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
    9ff0bb5b
highmem.c 3.52 KB