On Jun 12, 2008, at 5:03 PM, Dean Hildebrand wrote:
Hi Chuck,
Chuck Lever wrote:
Howdy Dean-
On Jun 10, 2008, at 2:54 PM, Dean Hildebrand wrote:
The motivation for this patch is improved WAN write performance
plus greater user control on the server of the TCP buffer values
(window size). The TCP window determines the amount of
outstanding data that a client can have on the wire and should be
large enough that a NFS client can fill up the pipe (the bandwidth
* delay product). Currently the TCP receive buffer size (used for
client writes) is set very low, which prevents a client from
filling up a network pipe with a large bandwidth * delay product.
Currently, the server TCP send window is set to accommodate the
maximum number of outstanding NFSD read requests (# nfsds *
maxiosize), while the server TCP receive window is set to a fixed
value which can hold a few requests. While these values set a TCP
window size that is fine in LAN environments with a small BDP, WAN
environments can require a much larger TCP window size, e.g.,
10GigE transatlantic link with a rtt of 120 ms has a BDP of approx
60MB.
Was the receive buffer size computation adjusted when support for
large transfer sizes was recently added to the NFS server?
Yes, it is based on the transfer size. So in the current code,
having a larger transfer size can improve efficiency PLUS help
create a larger possible TCP window. The issue seems to be that tcp
window, # of NFSDs, and transfer size are all independent variables
that need to be tuned individually depending on rtt, network
bandwidth, disk bandwidth, etc etc... We can adjust the last 2, so
this patch helps adjust the first (tcp window).
I have a patch to net/svc/svcsock.c that allows a user to manually
set the server TCP send and receive buffer through the sysctl
interface. to suit the required TCP window of their network
architecture. It adds two /proc entries, one for the receive
buffer size and one for the send buffer size:
/proc/sys/sunrpc/tcp_sndbuf
/proc/sys/sunrpc/tcp_rcvbuf
What I'm wondering is if we can find some algorithm to set the
buffer and window sizes *automatically*. Why can't the NFS server
select an appropriately large socket buffer size by default?
Since the socket buffer size is just a limit (no memory is
allocated) why, for example, shouldn't the buffer size be large for
all environments that have sufficient physical memory?
I think the problem there is that the only way to set the buffer
size automatically would be to know the rtt and bandwidth of the
network connection. Excessive numbers of packets can get dropped if
the TCP buffer is set too large for a specific network connection.
In this case, the window opens too wide and lets too many packets
out into the system, somewhere along the path buffers start
overflowing and packets are lost, TCP congestion avoidance kicks in
and cuts the window size dramatically and performance along with
it. This type of behaviour creates a sawtooth pattern for the TCP
window, which is less favourable than a more steady state pattern
that is created if the TCP buffer size is set appropriately.
Agreed it is a performance problem, but I thought some of the newer
TCP congestion algorithms were specifically designed to address this
by not closing the window as aggressively.
Once the window is wide open, then, it would appear that choosing a
good congestion avoidance algorithm is also important.
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.
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. 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). It does not show
whether the client and server are using TCP optimally. 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).
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.
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.
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?
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?
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.
The server cannot do just the same thing as the client since it
cannot just rely on the tcp sysctl's since it also needs to ensure
it has enough buffer space for each NFSD.
I agree the server's current logic is too conservative.
However, the server has an automatic load-leveling feature -- it can
close sockets if it notices it is running out of resources, and the
Linux server does this already. I don't think it would be terribly
harmful to overcommit the socket buffer space since we have such a
safety valve.
My goal with this patch is to provide users with the same
flexibility that the client has regarding tcp buffer sizes, but also
ensure that the minimum amount of buffer space that the NFSDs
require is allocated.
What is the formula you used to determine the value to poke into the
sysctl, btw?
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?
--
Chuck Lever
chuck[dot]lever[at]oracle[dot]com
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