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This patch address a serious performance issue in reading the TCP sockets table (/proc/net/tcp). Reading the full table is done by a number of sequential read operations. At each read operation, a seek is done to find the last socket that was previously read. This seek operation requires that the sockets in the table need to be counted up to the current file position, and to count each of these requires taking a lock for each non-empty bucket. The whole algorithm is O(n^2). The fix is to cache the last bucket value, offset within the bucket, and the file position returned by the last read operation. On the next sequential read, the bucket and offset are used to find the last read socket immediately without needing ot scan the previous buckets the table. This algorithm t read the whole table is O(n). The improvement offered by this patch is easily show by performing cat'ing /proc/net/tcp on a machine with a lot of connections. With about 182K connections in the table, I see the following: - Without patch time cat /proc/net/tcp > /dev/null real 1m56.729s user 0m0.214s sys 1m56.344s - With patch time cat /proc/net/tcp > /dev/null real 0m0.894s user 0m0.290s sys 0m0.594s Signed-off-by: Tom Herbert <therbert@google.com> Acked-by: Eric Dumazet <eric.dumazet@gmail.com> Signed-off-by: David S. Miller <davem@davemloft.net> |
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