Dean Hildebrand wrote:
We have a full picture of TCP. TCP is well known, there are lots of
papers/info on it, I have no doubt on what is occurring with TCP as I
have traces that clearly show what is happening. All documents and
information clearly state that the buffer size is a critical part of
improving TCP performance in the WAN. In addition, the congestion
control algorithm does NOT control the maximum size of the TCP window.
The CCA controls how quickly the window reaches the maximum size, what
happens when a packet is dropped and when to close the window. The only
item that controls the maximum size of the TCP window is the buffer
values that I want a sysctl to tweak (just to be in line with the
existing tcp buffer sysctls in Documentation/networking/ip-sysctl.txt)
IMO it's just plain broken that the application layer has to understand
and manage this detail about TCP.
What we don't have is a full picture of the other parts of transferring
data from client to server, e.g., Trond just fixed a bug with regards to
the writeback cache which should help write performance, that was an
unknown up until this point.
Multiple TCP Streams
===============
There is a really big downside to multiple TCP streams: you have
multiple TCP streams :) Each one has its own overhead, setup connection
cost, and of course TCP window. With a WAN rtt of 200 ms (typical
over satellite) and the current buffer size of 4MB, the nfs client would
need 50+ TCP connections to achieve the correct performance. That is a
lot of overhead when comparing it with simply following the standard TCP
tuning knowhow of increasing the buffer sizes.
I suspect that anyone operating NFS over a sat link would have an
already lowered performance expectation.
If the maximum window size defaults to, say, 16MB, and you have a
smaller RTT (which is typical of intercontinental 10GbE links which you
might be more willing to pump huge amounts of data over than a sat
link), you will need fewer concurrent connections to achieve optimal
performance, and that becomes more practical.
For networks with a much smaller BDP (like, say, MOST OF THEM :-) you
might be able to get away with only a few connections, or even one, if
we decide to use a larger default maximum window size.
There are plenty of advantages to having multiple connections between
client and server. The fact that it helps the large BDP case is just a
bonus.
The main documentation the show that multiple tcp streams helps over the
WAN is from GridFTP experiments. They go over the pos and neg of the
approach, but also talk about how tcp buffer size is also very
important. Multiple tcp streams is not a replacement for a proper
buffer size
(http://www.globus.org/alliance/publications/clusterworld/0904GridFinal.pdf)
Here's something else to consider:
TCP is likely not the right transport protocol for networks with a large
BDP. Perhaps SCTP, which embeds support for multiple streams in a
single connection, is better for this case... and what we really want to
do is create an SCTP-based transport capability for NFS. Or maybe we
really want to use iWARP over SCTP.
If you have documentation counteracting these experiments I would be
very interested to see them.
I think you are willfully misinterpreting my objection to your sysctl patch.
I never said your experiments are incorrect; they are valuable. My
point is that they don't demonstrate that this is a useful knob for our
most common use cases, and that it is the correct and only way to get
"good enough" performance for most common deployments of NFS. It helps
the large BDP case, but as you said, it doesn't make all the problems go
away there either.
Is it easy to get optimal results with this? How do admins evaluate the
results of changing this value? Is it easy to get bad results with it?
Can it result in bad behavior that results in problems for other users
of the network?
I also never said "use multiple connections but leave the buffer size
alone." I think we agree that a larger receive buffer size is a good
idea for the NFS server, in general. The question is whether allowing
admins to tune it is the most effective way to benefit performance for
our user base, or can we get away with using a more optimal but fixed
default size (which is simpler for admins to understand and for us to
maintain)?
Or are we just working around what is effectively a defect in TCP itself?
I think we need to look at the bigger picture, which contains plenty of
other interesting alternatives that may have larger benefit.
One Variable or Two
===============
I'd be happy with using a single variable for both the send and receive
buffers, but since we are essentially doing the same thing as the
net.ipv4.tcp_wmem/rmem variables, I think nfsd_tcp_max_mem would be more
in line with existing Linux terminology. (also, we are talking about
nfsd, not nfs, so I'd prefer to make that clear in the variable name)
Summary
=======
I'm providing you with all the information I have with regards to my
experiments with NFS and TCP. I agree that a better default is needed
and my patch allows further experimentation to get to that value. My
patch does not add modify current NFS behaviour. It changes a hard
coded value for the server buffer size to be a variable in /proc.
Blocking a method to modify this hard coded value means blocking further
experimentation to find a better default value. My patch is a first
step toward trying to find a good default tcp server buffer value.
Since what we really want to limit is the maximum size of the TCP
receive window, it would be more precise to change the name of the new
sysctl to something like nfs_tcp_max_window_size.
Another point is that setting the buffer size isn't always a
straightforward process. All papers I've read on the subject, and
my experience confirms this, is that setting tcp buffer sizes is
more of an art.
So having the server set a good default value is half the battle,
but allowing users to twiddle with this value is vital.
The uses the current buffer sizes in the code are as minimum
values, which the user cannot decrease. If the user sets a value
of 0 in either /proc entry, it resets the buffer size to the
default value. The set /proc values are utilized when the TCP
connection is initialized (mount time). The values are bounded
above by the *minimum* of the /proc values and the network TCP
sysctls.
To demonstrate the usefulness of this patch, details of an
experiment between 2 computers with a rtt of 30ms is provided
below. In this experiment, increasing the server
/proc/sys/sunrpc/tcp_rcvbuf value doubles write performance.
EXPERIMENT
==========
This experiment simulates a WAN by using tc together with netem
to add a 30 ms delay to all packets on a nfs client. The goal is
to show that by only changing tcp_rcvbuf, the nfs client can
increase write performance in the WAN. To verify the patch has
the desired effect on the TCP window, I created two tcptrace
plots that show the difference in tcp window behaviour before and
after the server TCP rcvbuf size is increased. When using the
default server tcpbuf value of 6M, we can see the TCP window top
out around 4.6 M, whereas increasing the server tcpbuf value to
32M, we can see that the TCP window tops out around 13M.
Performance jumps from 43 MB/s to 90 MB/s.
Hardware:
2 dual-core opteron blades
GigE, Broadcom NetXtreme II BCM57065 cards
A single gigabit switch in the middle
1500 MTU
8 GB memory
Software:
Kernel: Bruce's 2.6.25-rc9-CITI_NFS4_ALL-1 tree
RHEL4
NFS Configuration:
64 rpc slots
32 nfsds
Export ext3 file system. This disk is quite slow, I therefore
exported using async to reduce the effect of the disk on the back
end. This way, the experiments record the time it takes for the
data to get to the server (not to the disk).
# exportfs -v
/export
<world>(rw,async,wdelay,nohide,insecure,no_root_squash,fsid=0)
# cat /proc/mounts
bear109:/export /mnt nfs
rw,vers=3,rsize=1048576,wsize=1048576,namlen=255,hard,nointr,proto=tcp,timeo=600,retrans=2,sec=sys,mountproto=udp,addr=9.1.74.144
0 0
fs.nfs.nfs_congestion_kb = 91840
net.ipv4.tcp_congestion_control = cubic
Network tc Command executed on client:
tc qdisc add dev eth0 root netem delay 30ms
rtt from client (bear108) to server (bear109)
#ping bear109
PING bear109.almaden.ibm.com (9.1.74.144) 56(84) bytes of data.
64 bytes from bear109.almaden.ibm.com (9.1.74.144): icmp_seq=0
ttl=64 time=31.4 ms
64 bytes from bear109.almaden.ibm.com (9.1.74.144): icmp_seq=1
ttl=64 time=32.0 ms
TCP Configuration on client and server:
# Controls IP packet forwarding
net.ipv4.ip_forward = 0
# Controls source route verification
net.ipv4.conf.default.rp_filter = 1
# Do not accept source routing
net.ipv4.conf.default.accept_source_route = 0
# Controls the System Request debugging functionality of the kernel
kernel.sysrq = 0
# Controls whether core dumps will append the PID to the core
filename
# Useful for debugging multi-threaded applications
kernel.core_uses_pid = 1
# Controls the use of TCP syncookies
net.ipv4.tcp_syncookies = 1
# Controls the maximum size of a message, in bytes
kernel.msgmnb = 65536
# Controls the default maxmimum size of a mesage queue
kernel.msgmax = 65536
# Controls the maximum shared segment size, in bytes
kernel.shmmax = 68719476736
# Controls the maximum number of shared memory segments, in pages
kernel.shmall = 4294967296
### IPV4 specific settings
net.ipv4.tcp_timestamps = 0
net.ipv4.tcp_sack = 1
# on systems with a VERY fast bus -> memory interface this is the
big gainer
net.ipv4.tcp_rmem = 4096 16777216 16777216
net.ipv4.tcp_wmem = 4096 16777216 16777216
net.ipv4.tcp_mem = 4096 16777216 16777216
### CORE settings (mostly for socket and UDP effect)
net.core.rmem_max = 16777216
net.core.wmem_max = 16777216
net.core.rmem_default = 16777216
net.core.wmem_default = 16777216
net.core.optmem_max = 16777216
net.core.netdev_max_backlog = 300000
# Don't cache ssthresh from previous connection
net.ipv4.tcp_no_metrics_save = 1
# make sure we don't run out of memory
vm.min_free_kbytes = 32768
Experiments:
On Server: (note that the real tcp buffer size is double tcp_rcvbuf)
[root@bear109 ~]# echo 0 > /proc/sys/sunrpc/tcp_rcvbuf
[root@bear109 ~]# cat /proc/sys/sunrpc/tcp_rcvbuf
3158016
On Client:
mount -t nfs bear109:/export /mnt
[root@bear108 ~]# iozone -aec -i 0 -+n -f /mnt/test -r 1M -s 500M
...
KB reclen write
512000 1024 43252 umount /mnt
On server:
[root@bear109 ~]# echo 16777216 > /proc/sys/sunrpc/tcp_rcvbuf
[root@bear109 ~]# cat /proc/sys/sunrpc/tcp_rcvbuf
16777216
On Client:
mount -t nfs bear109:/export /mnt
[root@bear108 ~]# iozone -aec -i 0 -+n -f /mnt/test -r 1M -s 500M
...
KB reclen write
512000 1024 90396
The numbers you have here are averages over the whole run.
Performing these tests using a variety of record lengths and file
sizes (up to several tens of gigabytes) would be useful to see
where different memory and network latencies kick in.
Definitely useful, although I'm not sure how this relates to this
patch.
It relates to the whole idea that this is a valid and useful
parameter to tweak.
What your experiment shows is that there is some improvement when
the TCP window is allowed to expand. It does not demonstrate that
the *best* way to provide this facility is to allow administrators
to tune the server's TCP buffer sizes.
By definition of how TCP is designed, tweaking the send and receive
buffer sizes is a useful. Please see the tcp tuning guides in my
other post. I would characterize tweaking the buffers as a necessary
condition but not a sufficient condition to achieve good throughput
with tcp over long distances.
A single average number can hide a host of underlying sins. This
simple experiment, for example, does not demonstrate that TCP window
size is the most significant issue here.
I would say it slightly differently, that it demonstrates that it is
significant, but maybe not the *most* significant. There are many
possible bottlenecks and possible knobs to tweak. For example, I'm
still not achieving link speeds, so I'm sure there are other
bottlenecks that are causing reduced performance.
I think that's my basic point. We don't have the full picture yet.
There are benefits to adjusting the maximum window size, but as we
learn more it may turn out that we want an entirely different knob or
knobs.
It does not show that it is more or less effective to adjust the
window size than to select an appropriate congestion control
algorithm (say, BIC).
Any tcp cong. control algorithm is highly dependent on the tcp buffer
size. The choice of algorithm changes the behaviour when packets are
dropped and in the initial opening of the window, but once the window
is open and no packets are being dropped, the algorithm is
irrelevant. So BIC, or westwood, or highspeed might do better in the
face of dropped packets, but since the current receive buffer is so
small, dropped packets are not the problem. Once we can use the
sysctl's to tweak the server buffer size, only then is the choice of
algorithm going to be important.
Maybe my use of the terminology is imprecise, but clearly the
congestion control algorithm matters for determining the TCP window
size, which is exactly what we're discussing here.
It does not show whether the client and server are using TCP optimally.
I'm not sure what you mean by *optimally*. They use tcp the only way
they know how non?
I'm talking about whether they use Nagle, when they PUSH, how they use
the window (servers can close a window when they are busy, for
example), and of course whether they can or should use multiple
connections.
It does not expose problems related to having a single data stream
with one blocking head (eg SCTP can allow multiple streams over the
same connection; or better performance might be achieved with
multiple TCP connections, even if they allow only small windows).
Yes, using multiple tcp connections might be useful, but that doesn't
mean you wouldn't want to adjust the tcp window of each one using my
patch. Actually, I can't seem to find the quote, but I read somewhere
that achieving performance in the WAN can be done 2 different ways:
a) If you can tune the buffer sizes that is the best way to go, but
b) if you don't have root access to change the linux tcp settings
then using multiple tcp streams can compensate for small buffer sizes.
Andy has/had a patch to add multiple tcp streams to NFS. I think his
patch and my patch work in collaboration to improve wan performance.
Yep, I've discussed this work with him several times. This might be a
more practical solution than allowing larger window sizes (one reason
being the dangers of allowing the window to get too large).
While the use of multiple streams has benefits besides increasing the
effective TCP window size, only the client side controls the number of
connections. The server wouldn't have much to say about it.
This patch isn't trying to alter default values, or predict buffer
sizes based on rtt values, or dynamically alter the tcp window
based on dropped packets, etc, it is just giving users the ability
to customize the server tcp buffer size.
I know you posted this patch because of the experiments at CITI with
long-run 10GbE, and it's handy to now have this to experiment with.
Actually at IBM we have our own reasons for using NFS over the WAN. I
would like to get these 2 knobs into the kernel as it is hard to tell
customers to apply kernel patches....
It might also be helpful if we had a patch that made the server
perform better in common environments, so a better default setting
it seems to me would have greater value than simply creating a new
tuning knob.
I think there are possibly 2 (or more) patches. One that improves the
default buffer sizes and one that lets sysadmins tweak the value. I
don't see why they are mutually exclusive.
They are not. I'm OK with studying the problem and adjusting the
defaults appropriately.
The issue is whether adding this knob is the right approach to
adjusting the server. I don't think we have enough information to
understand if this is the most useful approach. In other words, it
seems like a band-aid right now, but in the long run it might be the
correct answer.
My patch is a first step towards allowing NFS into WAN environments.
Linux currently has sysctl values for the TCP parameters for exactly
this reason, it is impossible to predict the network environment of a
linux machine.
If the Linux nfs server isn't going to build off of the existing
Linux TCP values (which all sysadmins know how to tweak), then it
must allow sysadmins to tweak the NFS server tcp values, either using
my patch or some other related patch. I'm open to how the server tcp
buffers are tweaked, they just need to be able to be tweaked. For
example, if all tcp buffer values in linux were taken out of the
/proc file system and hardcoded, I think there would be a revolt.
I'm not arguing for no tweaking. What I'm saying is we should provide
knobs that are as useful as possible, and include metrics and clear
instructions for when and how to set the knob.
You've shown there is improvement, but not that this is the best
solution. It just feels like the work isn't done yet.
Would it be hard to add a metric or two with this tweak that would
allow admins to see how often a socket buffer was completely full,
completely empty, or how often the window size is being aggressively
cut?
So I've done this using tcpdump in combination with tcptrace. I've
shown people at citi how the tcp window grows in the experiment I
describe.
No, I mean as a part of the patch that adds the tweak, it should
report various new statistics that can allow admins to see that they
need adjustment, or that there isn't a problem at all in this area.
Scientific system tuning means assessing the problem, trying a change,
then measuring to see if it was effective, or if it caused more
trouble. Lather, rinse, repeat.
While we may not be able to determine a single optimal buffer size
for all BDPs, are there diminishing returns in most common cases for
increasing the buffer size past, say, 16MB?
Good question. It all depends on how much data you are transferring.
In order to fully open a 128MB tcp window over a very long WAN, you
will need to transfer at least a few gigabytes of data. If you only
transfer 100 MB at a time, then you will probably be fine with a 16
MB window as you are not transferring enough data to open the window
anyways. In our environment, we are expecting to transfer 100s of GB
if not even more, so the 16 MB window would be very limiting.
What about for a fast LAN?
The information you are curious about is more relevant to creating
better default values of the tcp buffer size. This could be useful,
but would be a long process and there are so many variables that
I'm not sure that you could pick proper default values anyways. The
important thing is that the client can currently set its tcp buffer
size via the sysctl's, this is useless if the server is stuck at a
fixed value since the tcp window will be the minimum of the client
and server's tcp buffer sizes.
Well, Linux servers are not the only servers that a Linux client
will ever encounter, so the client-side sysctl isn't as bad as
useless. But one can argue whether that knob is ever tweaked by
client administrators, and how useful it is.
Definitely not useless. Doing a google search for 'tcp_rmem' returns
over 11000 hits describing how to configure tcp settings. (ok, I
didn't review every result, but the first few pages of results are
telling) It doesn't really matter what OS the client and server use,
as long as both have the ability to tweak the tcp buffer size.
The number of hits may reflect the desperation that many have had over
the years to get better performance from the Linux NFS
implementation. These days we have better performance out of the box,
so there is less need for this kind of after-market tweaking.
I think we would be in a much better place if the client and server
implementations worked "well enough" in nearly any network or
environment. That's been my goal since I started working on Linux NFS
seven years ago.
What is an appropriate setting for a server that has to handle a mix
of local and remote clients, for example, or a client that has to
connect to a mix of local and remote servers?
Yes, this is a tricky one. I believe the best way to handle it is to
set the server tcp buffer to the MAX(local, remote) and then let the
local client set a smaller tcp buffer and the remote client set a
larger tcp buffer. The problem there is that then what if the local
client is also a remote client of another nfs server?? At this point
there seems to be some limitations.....
Using multiple connections solves this problem pretty well, I think.
--
Chuck Lever
chuck[dot]lever[at]oracle[dot]com
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