Date: Wed, 11 Apr 2012 15:02:14 +0000 (UTC) From: Luigi Rizzo <luigi@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r234137 - head/sys/dev/ixgbe Message-ID: <201204111502.q3BF2Eb3082893@svn.freebsd.org>
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Author: luigi Date: Wed Apr 11 15:02:14 2012 New Revision: 234137 URL: http://svn.freebsd.org/changeset/base/234137 Log: Enable prefetching of descriptors on the TX ring, using the same values as in the Intel driver 3.8.21 for linux. The fact that it is standard in the above driver suggests that it has no bad side effects. But of course there must be a reason for enabling features, not just "it does not harm", so here it is a good one: Prefetching enables full line rate even using a single queue (14.88 Mpps, compared to ~12 Mpps without prefetch). This in turn is terribly useful when one wants to schedule traffic. For obvious reasons the difference is only visible with netmap or other high speed solutions, but presumably the advantage should be in the order of a fraction of a microsecond when starting transmission on an empty queue. Discussed with Jack Vogel. MFC after: 1 week Modified: head/sys/dev/ixgbe/ixgbe.c Modified: head/sys/dev/ixgbe/ixgbe.c ============================================================================== --- head/sys/dev/ixgbe/ixgbe.c Wed Apr 11 14:54:06 2012 (r234136) +++ head/sys/dev/ixgbe/ixgbe.c Wed Apr 11 15:02:14 2012 (r234137) @@ -1143,6 +1143,14 @@ ixgbe_init_locked(struct adapter *adapte txdctl |= IXGBE_TXDCTL_ENABLE; /* Set WTHRESH to 8, burst writeback */ txdctl |= (8 << 16); + /* + * When the internal queue falls below PTHRESH (32), + * start prefetching as long as there are at least + * HTHRESH (1) buffers ready. The values are taken + * from the Intel linux driver 3.8.21. + * Prefetching enables tx line rate even with 1 queue. + */ + txdctl |= (16 << 0) | (1 << 8); IXGBE_WRITE_REG(hw, IXGBE_TXDCTL(i), txdctl); }
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