Re: [PATCH 0/1] SUNRPC: Add sysctl variables for server TCP snd/rcv buffer values

[Dean Hildebrand <seattleplus@gmail.com>]

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.

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.  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.  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.  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.

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.
>
> In addition, have you looked at network traces to see if the server's 
> TCP implementation is behaving optimally (or near optimally)?  Have 
> you tried using some of the more esoteric TCP congestion algorithms 
> available in 2.6 kernels?
I guess you are asking if I'm sure that I'm fixing the right problem?  
Nothing is broken in terms of the tcp implementation, it just requires 
bigger buffers to handle a larger BDP.  iperf, bbcp, etc all use the 
same tcp implementation and all work fine if giving a larger enough 
buffer size, so I know tcp is fine.  From reading WAN tuning papers, I 
know that setting a 3 MB server tcp buffer size (current rcvbuf default 
in linux server) is not sufficient for a BDP of, for example, 60 MB or 
more.  I've tried every tcp implementation available in the kernel at 
one point or another, but actually I've found bic to be the best in WAN 
environments since it is one of the most aggressive. 
>
> There are also fairly unsophisticated ways to add longer delays on 
> your test network, and turning up the latency knob would be a useful 
> test.
My experiment uses tc with netem to control the latency, so I can run 
any experiment, but I don't learn a lot beyond the experiment that I've 
presented.  Essentially, the bigger the BDP, the bigger your tcp buffers 
need to be.

The NFS client currently leaves tcp buffer sizes to the user, and I 
would prefer to do the same on the server via a sysctl.
Dean
>
> -- 
> Chuck Lever
> chuck[dot]lever[at]oracle[dot]com