On 08/17/2015 05:38 AM, Andrei Borzenkov wrote:
On Thu, Aug 13, 2015 at 4:59 PM, Josef Bacik <jba...@fb.com> 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. 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). The server
is definitely getting all of the ACK's,
(Sorry was traveling for Linux Plumbers.)
In packet trace you sent me there was almost certain ACK loss for the
segment 20801001- 20805881 (frame 19244). Note that here recovery was
rather fast - server started retransmission after ~0.5sec. It is
unlikely lost packet from server - next ACK from GRUB received by
server was for 20803441, which means it actually got at least initial
half of this segment. Unfortunately some packets are missing in
capture (even packets *from* server), which makes it harder to
interpret. After this server went down to 512 segment size and
everything went more or less well, until frame 19949. Here the server
behavior is rather interesting. It starts retransmission with initial
timeout ~6sec, even though it received quite a lot of DUP ACKs; and
doubling it every time until it hits GRUB timeout (~34 seconds).
Yeah that's the normal re-transmission timeout. This tcpdump was on a
non-patched grub. We only sent 3 dup acks, we have the dup ack counter
stuff set to like 13 or something like that so we have to get a lot
before it triggers the dup ack retransmit logic.
Note the difference in behavior between the former and the latter. Did
you try to ask on Linux networking list why they are so different?
I'll run it by our networking guys when they show up.
OTOH GRUB probably times out too early. Initial TCP RFC suggests 5
minutes general timeout and RFC1122 - at least 100 seconds. It would
be interesting to increase connection timeout to see if it recovers.
You could try bumping GRUB_NET_TRIES to 82 which result in timeout
slightly over 101 sec.
Also it seems that huge window may aggravate the issue. According to
trace, 10K is enough to fill pipe and you set it to 1M. It would be
interesting to see the same with default windows size.
Oh yeah the problem doesn't happen with a normal window size, it's only
with the giant window size. I'm not sure where you are getting the 10k
number, believe me if I could have gotten around this by just jacking up
the normal window size I would have done it. When I set it to the max
(64k I think?) I get a transfer rate of around 200 kb/s, which is not
fast enough to pull down our 250mb image. With the 1mb window I get 5.5
mb/s, so there is a real benefit to the giant window. Thanks,
Josef
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