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tcp: allow dctcp alpha to drop to zero

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If alpha is strictly reduced by alpha >> dctcp_shift_g and if alpha is less
than 1 << dctcp_shift_g, then alpha may never reach zero. For example,
given shift_g=4 and alpha=15, alpha >> dctcp_shift_g yields 0 and alpha
remains 15. The effect isn't noticeable in this case below cwnd=137, but
could gradually drive uncongested flows with leftover alpha down to
cwnd=137. A larger dctcp_shift_g would have a greater effect.

This change causes alpha=15 to drop to 0 instead of being decrementing by 1
as it would when alpha=16. However, it requires one less conditional to
implement since it doesn't have to guard against subtracting 1 from 0U. A
decay of 15 is not unreasonable since an equal or greater amount occurs at
alpha >= 240.

Signed-off-by: Andrew G. Shewmaker <agshew@gmail.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Andrew Shewmaker 2015-10-18 21:59:08 -07:00 committed by David S. Miller
parent ab997ad408
commit c80dbe0461
1 changed files with 1 additions and 1 deletions
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net/ipv4/tcp_dctcp.c
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@ -209,7 +209,7 @@ static void dctcp_update_alpha(struct sock *sk, u32 flags)
/* alpha = (1 - g) * alpha + g * F */
alpha -= alpha >> dctcp_shift_g;
alpha -= min_not_zero(alpha, alpha >> dctcp_shift_g);
if (bytes_ecn) {
/* If dctcp_shift_g == 1, a 32bit value would overflow
* after 8 Mbytes.