<div dir="ltr">I'm having trouble understanding the concept of readahead in an HTTP context.<div><br></div><div>You are using the malloc cache storage, right?</div></div><div class="gmail_extra"><br clear="all"><div><div class="gmail_signature" data-smartmail="gmail_signature"><div dir="ltr"><div>-- <br></div>Guillaume Quintard<br></div></div></div>
<br><div class="gmail_quote">On Thu, Jul 6, 2017 at 7:15 PM, John Salmon <span dir="ltr"><<a href="mailto:John.Salmon@deshawresearch.com" target="_blank">John.Salmon@deshawresearch.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000">
Thanks for your suggestions. <br>
<br>
One more detail I didn't mention: Roughly speaking, the client is
doing "read ahead", but it only reads ahead by a limited amount
(about 4 blocks, each of 128KiB). The surprising behavior is that
when four readahead threads are allowed to run concurrently their
aggregate throughput is much lower than when all the readaheads are
serialized through a single thread. <br>
<br>
Traces (with strace and/or tcpdump) show frequent stalls of roughly
200ms where nothing seems to move across the channel and all
client-side system calls are waiting. 200ms is suspiciously close
to the linux 'rto_min' parameter, which was the first thing that led
me to suspect TCP incast collapse. We get some improvement by
reducing rto_min on the server, and we also get some improvement by
reducing SO_RCVBUF in the client. But as I said, both have
tradeoffs, so I'm interested if anyone else has encountered or
overcome this particular problem.<br>
<br>
I do not see the dropoff from single-thread to multi-thread when I
client and server on the same host. I.e., I get around 500MB/s with
one client and roughly the same total bandwidth with multiple
clients. I'm sure that with some tuning, the 500MB/s could be
improved, but that's not the issue here.<br>
<br>
Here are the ethtool reports:<br>
<br>
On the client:<br>
drdws0134$ ethtool eth0<br>
Settings for eth0:<br>
Supported ports: [ TP ]<br>
Supported link modes: 10baseT/Half 10baseT/Full <br>
100baseT/Half 100baseT/Full <br>
1000baseT/Full <br>
Supported pause frame use: No<br>
Supports auto-negotiation: Yes<br>
Advertised link modes: 10baseT/Half 10baseT/Full <br>
100baseT/Half 100baseT/Full <br>
1000baseT/Full <br>
Advertised pause frame use: No<br>
Advertised auto-negotiation: Yes<br>
Speed: 1000Mb/s<br>
Duplex: Full<br>
Port: Twisted Pair<br>
PHYAD: 1<br>
Transceiver: internal<br>
Auto-negotiation: on<br>
MDI-X: on (auto)<br>
Cannot get wake-on-lan settings: Operation not permitted<br>
Current message level: 0x00000007 (7)<br>
drv probe link<br>
Link detected: yes<br>
drdws0134$ <br>
<br>
On the server:<br>
<br>
$ ethtool eth0<br>
Settings for eth0:<br>
Supported ports: [ TP ]<br>
Supported link modes: 1000baseT/Full <br>
10000baseT/Full <br>
Supported pause frame use: No<br>
Supports auto-negotiation: No<br>
Advertised link modes: Not reported<br>
Advertised pause frame use: No<br>
Advertised auto-negotiation: No<br>
Speed: 10000Mb/s<br>
Duplex: Full<br>
Port: Twisted Pair<br>
PHYAD: 0<br>
Transceiver: internal<br>
Auto-negotiation: off<br>
MDI-X: Unknown<br>
Cannot get wake-on-lan settings: Operation not permitted<br>
Cannot get link status: Operation not permitted<br>
$ <br><div><div class="h5">
<br>
<br>
<div class="m_-5994494751137666232moz-cite-prefix">On 07/06/2017 03:08 AM, Guillaume
Quintard wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">Two things: do you get the same results when the
client is directly on the Varnish server? (ie. not going through
the switch) And is each new request opening a new connection?</div>
<div class="gmail_extra"><br clear="all">
<div>
<div class="m_-5994494751137666232gmail_signature" data-smartmail="gmail_signature">
<div dir="ltr">
<div>-- <br>
</div>
Guillaume Quintard<br>
</div>
</div>
</div>
<br>
<div class="gmail_quote">On Thu, Jul 6, 2017 at 6:45 AM, Andrei
<span dir="ltr"><<a href="mailto:lagged@gmail.com" target="_blank">lagged@gmail.com</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div dir="ltr">Out of curiosity, what does ethtool show for
the related nics on both servers? I also have Varnish on a
10G server, and can reach around 7.7Gbit/s serving
anywhere between 6-28k requests/second, however it did
take some sysctl tuning and the westwood TCP congestion
control algo</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">
<div>
<div class="m_-5994494751137666232h5">On Wed, Jul 5, 2017 at 3:09 PM, John
Salmon <span dir="ltr"><<a href="mailto:John.Salmon@deshawresearch.com" target="_blank">John.Salmon@deshawresearch.co<wbr>m</a>></span>
wrote:<br>
</div>
</div>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div>
<div class="m_-5994494751137666232h5">
<div bgcolor="#FFFFFF" text="#000000"> I've been
using Varnish in an "intranet" application. The
picture is roughly:<br>
<br>
origin <-> Varnish <-- 10G channel
---> switch <-- 1G channel --> client<br>
<br>
The machine running Varnish is a
high-performance server. It can<br>
easily saturate a 10Gbit channel. The machine
running the client is a<br>
more modest desktop workstation, but it's fully
capable of saturating<br>
a 1Gbit channel.<br>
<br>
The client makes HTTP requests for objects of
size 128kB.<br>
<br>
When the client makes those requests serially,
"useful" data is<br>
transferred at about 80% of the channel
bandwidth of the Gigabit<br>
link, which seems perfectly reasonable.<br>
<br>
But when the client makes the requests in
parallel (typically<br>
4-at-a-time, but it can vary), *total*
throughput drops to about 25%<br>
of the channel bandwidth, i.e., about
30Mbyte/sec.<br>
<br>
After looking at traces and doing a fair amount
of experimentation, we<br>
have reached the tentative conclusion that we're
seeing "TCP Incast<br>
Throughput Collapse" (see references below)<br>
<br>
The literature on "TCP Incast Throughput
Collapse" typically describes<br>
scenarios where a large number of servers
overwhelm a single inbound<br>
port. I haven't found any discussion of incast
collapse with only one<br>
server, but it seems like a natural consequence
of a 10Gigabit-capable<br>
server feeding a 1-Gigabit downlink.<br>
<br>
Has anybody else seen anything similar? With
Varnish or other single<br>
servers on 10Gbit to 1Gbit links.<br>
<br>
The literature offers a variety of mitigation
strategies, but there are<br>
non-trivial tradeoffs and none appears to be a
silver bullet.<br>
<br>
If anyone has seen TCP Incast Collapse with
Varnish, were you able to work<br>
around it, and if so, how?<br>
<br>
Thanks,<br>
John Salmon<br>
<br>
References:<br>
<br>
<a class="m_-5994494751137666232m_-5687506660243993301m_5374091370556894899moz-txt-link-freetext" href="http://www.pdl.cmu.edu/Incast/" target="_blank">http://www.pdl.cmu.edu/Incast/</a><br>
<br>
Annotated Bibliography in:<br>
<a class="m_-5994494751137666232m_-5687506660243993301m_5374091370556894899moz-txt-link-freetext" href="https://lists.freebsd.org/pipermail/freebsd-net/2015-November/043926.html" target="_blank">https://lists.freebsd.org/pipe<wbr>rmail/freebsd-net/2015-Novembe<wbr>r/043926.html</a><span class="m_-5994494751137666232m_-5687506660243993301HOEnZb"><font color="#888888"><br>
<br>
<div class="m_-5994494751137666232m_-5687506660243993301m_5374091370556894899moz-signature">--
<br>
<b>.</b></div>
</font></span></div>
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