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
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