Limits on jumbo mbuf cluster allocation

[Rick Macklem]

Andre Oppermann wrote:
> On 10.03.2013 03:22, Rick Macklem wrote:
> > Garett Wollman wrote:
> >> Also, it occurs to me that this strategy is subject to livelock. To
> >> put backpressure on the clients, it is far better to get them to
> >> stop
> >> sending (by advertising a small receive window) than to accept
> >> their
> >> traffic but queue it for a long time. By the time the NFS code gets
> >> an RPC, the system has already invested so much into it that it
> >> should
> >> be processed as quickly as possible, and this strategy essentially
> >> guarantees[1] that, once those 2 MB socket buffers start to fill
> >> up,
> >> they
> >> will stay filled, sending latency through the roof. If nfsd didn't
> >> override the usual socket-buffer sizing mechanisms, then sysadmins
> >> could limit the buffers to ensure a stable response time.
> >>
> >> The bandwidth-delay product in our network is somewhere between
> >> 12.5
> >> kB and 125 kB, depending on how the client is connected and what
> >> sort
> >> of latency they experience. The usual theory would suggest that
> >> socket buffers should be no more than twice that -- i.e., about 256
> >> kB.
> >>
> > Well, the code that uses sb_max_adj wasn't written by me (I just
> > cloned
> > it for the new server). In the author's defence, I believe SB_MAX
> > was 256K when
> > it was written. It was 256K in 2011. I think sb_max_adj was used
> > because
> > soreserve() fails for a larger value and the code doesn't check for
> > such a failure.
> > (Yea, it should be fixed so that it checks for a failure return from
> > soreserve().
> >   I did so for the client some time ago.;-)
> 
> We have TCP sockbuf size autotuning for some time now. So explicitly
> setting the size shouldn't be necessary anymore.
> 
Ok. Is it possible for the size to drop below the size of the largest RPC?
(Currently a little over 64K and hopefully a little over 128K soon.)

I'm thinking of the restriction in sosend_generic() where it won't allow a
request greater than sb_hiwat to be added to the send queue. (It is passed
in as an mbuf list via the "top" argument, which makes "atomic" true, I think?)

The soreserve() calls were done in the old days to make sure sb_hiwat was
big enough that sosend() wouldn't return EMSGSIZE.
(I'll take a look at the code and try to see if/when sb_hiwat gets autotuned.)

> > Just grep for sb_max_adj. You'll see it sets a variable called
> > "siz".
> > Make "siz" whatever you want (256K sounds like a good guess). Just
> > make
> > sure it isn't > sb_max_adj.
> >
> > The I/O sizes are limited to MAXBSIZE, which is currently 64Kb,
> > although
> > I'd like to increase that to 128Kb someday soon. (As you note below,
> > the
> > largest RPC is slightly bigger than that.)
> >
> > Btw, net.inet.tcp.{send/recv}buf_max are both 2Mbytes, just like
> > sb_max,
> > so those don't seem useful in this case?
> 
> These are just the limits for auto-tuning.
> 
> > I'm no TCP guy, so suggestions w.r.t. how big soreserve() should be
> > set
> > are welcome.
> 
> I'd have to look more at the NFS code to see what exactly is going on
> and what the most likely settings are going to be. Won't promise any
> ETA though.
> 
Basically an RPC request/reply is an mbuf list where its size can be
up to MAXBSIZE + a hundred bytes or so. (64Kb+ --> 128Kb+ soon)

These need to be queued for sending without getting EMSGSIZE back.

Then, if the mount is for a high bandwidth WAN, it would be nice if
the send window allows several of these to be "in flight" (not yet
acknowledged) so that the "bit pipe" can be kept full (use the
available bandwidth). These could be read-aheads/write-behinds or
requests for other processes/threads in the client.
For example:
- with a 128Kbyte MAXBSIZE and a read-ahead of 15, it would be possible
  to have 128 * 1024 * 16 bytes on the wire, if the TCP window allows
  that. (This would fill a 1Gbps network with a 20msec rtt, if I got
  my rusty math correct. It is rtt and not the time for a packet to
  go in one direction, since the RPC replies need to get back to the
  client before it will do any more reads.) This sounds like the
  upper bound of the current setup, given the 2Mbyte setting for
  net.inet.tcp.sendbuf_max, I think?
  (Yes, I know most use NFS over a LAN, but it would be nice if it
   can work well enough over a WAN to be useful.)
- for a fast LAN, obviously the rtt is much lower, so the limit can
  be a lot lower. However, I'm not sure that there is an advantage
  w.r.t. NFS for this. So long as the client sees that it can't send
  more RPCs once several are queued for the server, it won't cause a
  "congestion collapse" for the server.
  I think the large window might make Garrett's case worse, since it seems that
  he is running out of mbuf clusters (actually the ability to allocate
  more jumbo ones), but that seems to be a fairly specific resource
  issue for his case caused in part by the fact the network interface
  is using so many of them.
  In other words, I'm not sure a generic NFS fix would make sense for
  this specific case.

Hope this helps and didn't just muddy the waters, rick


> --
> Andre
> 
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