Re: [RFC,PATCH 4/14] knfsd: has_wspace per transport

[Tom Tucker <tom@opengridcomputing.com>]

On 5/22/07 9:32 PM, "Greg Banks" <gnb@sgi.com> wrote:

> On Tue, May 22, 2007 at 12:34:23PM -0500, Tom Tucker wrote:
>> On Tue, 2007-05-22 at 21:16 +1000, Greg Banks wrote:
>>> 
>>> The wspace management code in knfsd is designed to avoid having
>>> nfsds ever block in the network stack; I don't see how the NFS/RDMA
>>> code achieves that.  Or is there something I've missed?
>>> 
>> 
>> You're dead on. We should implement wspace properly. I'll look into
>> fixing this. 
> 
> Great.
> 
>> The bulk of the stats were exported in order to evaluate different I/O
>> models. For example, do we poll some number of times on an SQ stall? How
>> about an RQ stall, etc... I think many of these stats should be removed.
>> I'd appreciate feedback on which ones we should retain.
> 
> Actually I quite like the stats, although the reap_[sr]q_at_* counters
> don't seem to be used.  For rdma_stat_rq_prod, I would have had a
> counter increment where wc.status reports an error, thus futzing with
> the global cacheline only once in the normal success path.
> 
> But I think it would be better to report them through the existing
> /proc/net/rpc/nfsd mechanism, i.e. add an RDMA-specific call to
> svc_seq_show() and add your new counters to struct svc_stat.
> 

I need to do a little clean up (e.g. atomic_inc()) and then I'll add them as
suggested. If there are dissenters, please speak now...

> 
>> What's happening here is that you're pounding the server with NFS WRITE
>> and recvfrom is dutifully queuing another thread to process the next RPC
>> in the RQ, which happens to require another RDMA READ and another wait
>> and so on. As you point out, this is not productive and quickly consumes
>> all your threads -- Ugh.
> 
> Exactly.
> 
>> A simple optimization to the current design for this case is that if the
>> RPC contains a read-list don't call svc_sock_enqueue until _after_ the
>> RDMA READ completes.
> 
> I assume you meant "don't release the SK_BUSY bit", because
> svc_sock_enqueue will be called from the dto tasklet every time
> a new call arrives whether or not we've finished the RDMA READ.
> 

Right. Good point. I might have spent a few hours debugging that one ;-)

>> The current thread will wait, but the remaining
>> threads (127 in your case) will be free to do other work for other mount
>> points. 
>> 
>> This only falls over in the limit of 128 mounts all pounding the server
>> with writes. Comments?
> 
> A common deployment scenario I would expect to see would have at
> least twice as many clients as threads, and SGI have successfully
> tested scenarios with a 16:1 client:thread ratio.  So I don't think
> we can solve the problem in any way which ties down a thread for
> up to 6 seconds per client, because there won't be enough threads
> to go around.

Agreed. But isn't that the 100 dead adapter scenario? Someone has to wait.
Who? 

> 
> The other case that needs to work is a single fat client trying to
> throw as many bits as possible down the wire.  For this case your
> proposal would limit the server to effectively serving WRITE RPCs
> serially, 

Not entirely. The approach would overlap the RPC fetch of the next WRITE
with the response to the previous.

>... with what I presume would be a catastrophic effect on
> write performance.  What you want is for knfsd to be able to throw
> as many threads at the single fat client as possible.
> 
> The knfsd design actually handles both these extreme cases quite
> well today with TCP sockets.  The features which I think are key
> to providing this flexibility are:
> 
>  * a pool of threads whose size is only loosely tied to the number
>    of clients
> 
>  * threads are not tied to clients, they compete for individual
>    incoming RPCs
> 
>  * threads take care to avoid blocking on the network
> 
> The problem we have achieving the last point for NFS/RDMA is that
> RDMA READs are fundamentally neither a per-svc_sock nor a per-thread
> concept.  To achieve parallelism you want multiple sequences of RDMA
> READs proceeding per svc_sock.  To avoid blocking threads you want a
> thread to be able to abandon a sequence of RDMA READs partway through
> and go back to it's main loop.  Doing both implies a new object to hold
> the state for a single sequence of RDMA READs for a single WRITE RPC.
> 
> So I think the "right" way to handle RDMA READs is to define a
> new struct, for argument's sake "struct svc_rdma_read", containing
> some large fraction of the auto variables currently in the function
> rdma_read_xdr() and some from it's caller.  When an RPC with a read
> list arrives, allocate a struct svc_rdma_read and add it to a list
> of such structs on the corresponding struct svcxprt_rdma.  Issue some
> READ WRs, until doing so would block.  When the completions for issued
> WRs arrive, mark the struct svc_rdma_read, instead of waking the
> nfsd as you do now, set SK_DATA on the svcxprt_rdma and enqueue it.
> When an nfsd later calls svc_rdma_recvfrom(), first check whether any
> of the struct svc_rdma_reads in flight have progressed, and depending
> on whether they're complete either issue some more READ WRs or finalise
> the struct svc_rdma_read by setting up the thread's page array.
> 
> In other words, invert the rdma_read_xdr() logic so nfsd return
> to the main loop instead of blocking.
> 
> Unfortunately it's kind of a major change.  Thoughts?
> 

Ok, I love it and I hate it :-)  This is the consolidated waiter strategy
that solves all the problems...except ...does this expose any read/write
ordering issues at the client? Couldn't the client issue a write followed by
a read and get the original data?

That's the hate it part. If we need to decide which requests are allowed to
proceed on a QP with outstanding READ WR, things get messy quick.

Is there no such requirement?

>>> And every now and again something goes awry in
>>> IB land and each thread ends up waiting for 6 seconds or more.
>> 
>> Yes. Note that when this happens, the connection gets terminated.
> 
> Indeed.  Meanwhile, for the last 6 seconds all the nfsds are
> blocked waiting for the one client, and none of the other clients
> are getting any service.

We need to think about this. A dead adapter causes havoc.

> 
> Greg.



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