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Date: Fri, 16 Feb 2007 15:02:53 -0500 From: Elad Lahav <elad_lahav@...rs.sourceforge.net> To: netdev@...r.kernel.org Subject: ip_append_page and the socket send buffer I wrote a function that is equivalent to udp_sendmsg, but uses ip_append_page to attach data to an skb. The function is implemented as follows: 1. Allocate a page and copy the given data to that page 2. Set up routing and cork the socket 3. Call ip_append_data to create an initial skb (with data length set to 0) 4. Call ip_append_page with the allocated page 5. Call udp_push_pending_frames to send the packet The function works correctly. Packets are generated and sent as expected: this was verified by looking at the packet contents on the receiving machine. However, under load, there is a significant difference in the behaviour of udp_sendmsg, compared with my function. The problem is that the socket send buffer (wmem_alloc) quickly grows beyond its upper limit (which is 131071 by default). This results in numerous failures of ip_append_data with EAGAIN, degrading performance considerably. udp_sendmsg, on the other hand, keeps wmem_alloc in a much smaller range under the same load. Two notes: 1. Modifying the upper limit to 524287 solved the problem completely (regardless of the load) 2. The same thing happens with multiple calls to ip_append_data (e.g., if I want to copy the data in two sections), so it is not a problem with ip_append_page. This leads me o believe that the problem lies with Scatter/Gather I/O. Any thoughts? Elad - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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