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: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
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