Implement headroom by moving packet around #481
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Related to #407 |
Instead of mutating a pointer inside "struct packet" to do cheap shiftleft/shiftright, this patch instead moves around the packet pointer itself. The only performance testing I did was "snabb lwaftr bench", where it appears to be neutral-to-slight-win. I am hoping that the Hydra/R scientists can help me out on this one. This change to the packet structure does have a knock-on effect, in two significant ways. One is that the shiftleft, shiftright, and prepend functions now return a new packet pointer and silently corrupt the old one (because probably they shifted around the data but still had to update the new "length" pointer). That's somewhat OK though. The problem comes in the datagram library, which sometimes wants to work on a user-supplied packet. The sequence was: local dgram = datagram:new(pkt) dgram:push(....) dgram:free() ... foo(pkt) This pattern is no longer valid :( The reason is that the datagram methods that mutate a packet often do so by packet.prepend(). The datagram library updates its internal packet pointer, but the external one that was initially passed in is no longer valid. So much of this patch is visiting (hopefully) all of the uses/users and updating them.
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I'm OK with the patch. I wait for others reviews/comments. |
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Let's wait on this one until merging in snabbco#1034 and snabbco#1032. |
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Because an equivalent change has landed on next, I'll land this via a merge from next instead. Closing this PR (though I looked at the diff here to see what I would need to do for the merge). |
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Instead of mutating a pointer inside "struct packet" to do cheap
shiftleft/shiftright, this patch instead moves around the packet
pointer itself.
The only performance testing I did was "snabb lwaftr bench", where it
appears to be neutral-to-slight-win. I am hoping that the Hydra/R
scientists can help me out on this one.
This change to the packet structure does have a knock-on effect, in
two significant ways. One is that the shiftleft, shiftright, and
prepend functions now return a new packet pointer and silently
corrupt the old one (because probably they shifted around the data but
still had to update the new "length" pointer). That's somewhat OK
though.
The problem comes in the datagram library, which sometimes wants
to work on a user-supplied packet. The sequence was:
local dgram = datagram:new(pkt)
dgram:push(....)
dgram:free()
...
foo(pkt)
This pattern is no longer valid :( The reason is that the datagram
methods that mutate a packet often do so by packet.prepend(). The
datagram library updates its internal packet pointer, but the external
one that was initially passed in is no longer valid. So much of this
patch is visiting (hopefully) all of the uses/users and updating them.