On 08/19/2014 02:06 AM, Thierry Reding wrote:
On Mon, Aug 18, 2014 at 01:01:17PM -0500, Joe Hershberger wrote:
Hi Thierry,
On Mon, Aug 18, 2014 at 1:45 AM, Thierry Reding <thierry.red...@gmail.com>
wrote:
From: Thierry Reding <tred...@nvidia.com>
It's not unusual for DHCP servers to take a couple hundred milliseconds
to respond to DHCP discover messages. One possible reason for the delay
can be that the server checks (typically using an ARP request) that the
IP it's about to hand out isn't in use yet. To make matters worse, some
servers may also queue up requests and process them sequentially, which
can cause excessively long delays if clients retry too fast.
Commit f59be6e850b3 ("net: BOOTP retry timeout improvements") shortened
the retry timeouts significantly, but the BOOTP/DHCP implementation in
U-Boot doesn't handle that well because it will ignore incoming replies
to earlier requests. In one particular setup this increases the time it
takes to obtain a DHCP lease from 630 ms to 8313 ms.
This commit attempts to fix this in two ways. First it increases the
initial retry timeout from 10 ms to 250 ms to give DHCP servers some
more time to respond. At the same time a cache of outstanding DHCP
request IDs is kept so that the implementation will know to continue
transactions even after a retransmission of the DISCOVER message. The
maximum retry timeout is also increased from 1 second to 2 seconds. An
ID cache of size 4 will keep DHCP requests around for 8 seconds (once
the maximum retry timeout has been reached) before dropping them. This
should give servers plenty of time to respond. If it ever turns out
that this isn't enough, the size of the cache can easily be increased.
With this commit the DHCP lease on the above-mentioned setup still takes
longer (1230 ms) than originally, but that's an acceptable compromise to
improve DHCP lease acquisition time for a broader range of setups.
To make it easier to benchmark DHCP in the future, this commit also adds
the time it took to obtain a lease to the final "DHCP client bound to
address x.x.x.x" message.
void BootpReset(void)
{
+ bootp_num_ids = 0;
BootpTry = 0;
bootp_start = get_timer(0);
- bootp_timeout = 10;
+ bootp_timeout = 250;
What is the impact on the time it takes for you to get an address assigned
in your environment with the rest of this patch, but without this change of
the initial timeout? It seems like it shouldn't impact you negatively but
will still help Stephen's original case.
Leaving the initial timeout at 10 ms increases the time to get a lease
from 1230 ms to 5350 ms. I suspect the reason for that is that the DHCP
server will queue an ARP ping for each request and process them
sequentially. Each of those takes about 600 ms and I see U-Boot sending
out a total of 9 broadcasts. So that's 5400 ms delay only due to the
requests that haven't been answered. The 8th broadcast is probably the
one that U-Boot receives a response for, hence explaining the 5350 ms.
Even the 250 ms timeout will make things a lot better for Stephen's use
case. Given the earlier discussion it seems like the DHCP server in his
network doesn't check for existing users of an IP address using ARP
pings and replies fairly quickly, so I suspect the whole process to take
around 300 ms for him. That's still a lot better than the 1230 ms that
it takes on my setup (which used to be somewhere around 630 ms before
Stephen's original patch).
I'm pretty sure I have seen ARP pings, although I guess the timeout on
my DHCP server must be much more reasonable, since the whole DHCP
process completes for me on the first request U-Boot sends.
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