On 08/13/2015 01:13 PM, Andrei Borzenkov wrote:
On 13.08.2015 16:59, Josef Bacik wrote:
On 08/13/2015 04:19 AM, Andrei Borzenkov wrote:
On Wed, Aug 12, 2015 at 6:16 PM, Josef Bacik <jba...@fb.com> wrote:
While adding tcp window scaling support I was finding that I'd get
some packet
loss or reordering when transferring from large distances and grub
would just
timeout. This is because we weren't ack'ing when we got our OOO
packet, so the
sender didn't know it needed to retransmit anything, so eventually it
would fill
the window and stop transmitting, and we'd time out. Fix this by
ACK'ing when
we don't find our next sequence numbered packet. With this fix I no
longer time
out. Thanks,
I have a feeling that your description is misleading. Patch simply
sends duplicated ACK, but partner does not know what has been received
and what has not, so it must wait for ACK timeout anyway before
retransmitting. What this patch may fix would be lost ACK packet
*from* GRUB, by increasing rate of ACK packets it sends. Do you have
packet trace for timeout case, ideally from both sides simultaneously?
The way linux works is that if you get <configurable amount> of DUP
ack's it triggers a retransmit.
Do you have pointers to documentation and code?
The tcp_reordering systctl allows you to set how many DUP acks you get
before retransmitting, you can see the comment above the function
tcp_time_to_recover in the kernel. With no SACK support we rely on
getting a certain number of DUP ACKs before retransmitting, as we could
get the out of order packets we want in time and not have to retransmit.
I only have traces from the server
since tcpdump doesn't work in grub (or if it does I don't know how to do
it).
GRUB does not have tcpdump, but your switch quite likely has port
mirroring.
Big comapny, big datacenters etc, etc. I'm a file system developer, you
are lucky I know how to spell tcpdump to begin with ;). The tcpdump on
the server side supports my hypothesis, we send lots and lots of stuff,
the grub box starts falling behind in it's ACK responses because it's
waiting for the next SEQ packet to come in, it ACK's when it does
finally come in with the new next expected SEQ, and this degrades to the
point where the sender has maxed out its send window and the grub box
either has lost or has yet to receive the next packet it is waiting for
and times out. I can say for sure that we aren't getting the next
packet we are looking for while getting a bunch of others just from my
instrumentation on the grub side, I _can't_ say for sure if it is just
simple re-ordering or packet loss somewhere. With this patch we're
definitely getting all of the DUP ACK's, at least there doesn't appear
to be any missing in the range (like I see DUP ACK #1-#300 all in a row,
not missing anybody.)
If you want I can change the commit log to say something like
"If we get an out of order packet we still need to ACK with the expected
SEQ number so the sender knows we haven't received that packet yet and
may need a retransmission."
To clear up any ambiguity. Thanks,
Josef
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