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

[Tom Tucker <tom@opengridcomputing.com>]

On Tue, 2007-05-22 at 21:16 +1000, Greg Banks wrote:
> On Fri, May 18, 2007 at 08:33:54AM -0500, Tom Tucker wrote:
> > On Fri, 2007-05-18 at 14:05 +1000, Greg Banks wrote:
> > > On Thu, May 17, 2007 at 08:30:39PM +1000, Neil Brown wrote:
> > > >  Do that mean that RDMA will never
> > > > reject a write due to lack of space? 
> > > 
> > > No, it means that the current RDMA send code will block waiting
> > > for space to become available.  That's right, nfsd threads block on
> > > the network.  Steel yourself, there's worse to come.
> > > 
> > 
> > Uh... Not really. The queue depths are designed to match credits to
> > worst case reply sizes. In the normal case, it should never have to
> > wait. The wait is to catch the margins in the same way that a kmalloc
> > will wait for memory to become available. 
> 
> This is news to me, but then I just read that code not write it ;-)

Uh oh, I'm about to get schooled aren't I...

> I just poked around in your GIT tree and AFAICT the software queue
> depth limits are set by this logic:
> 
>     743 static int
>     744 svc_rdma_accept(struct svc_rqst *rqstp)
>     745 {
>     ...
>     775         ret = ib_query_device(newxprt->sc_cm_id->device, &devattr);
>     ...
>     788         newxprt->sc_max_requests = min((size_t)devattr.max_qp_wr,
>     789                                    (size_t)svcrdma_max_requests);
>     790         newxprt->sc_sq_depth = RPCRDMA_SQ_DEPTH_MULT * newxprt->sc_max_requests;
> 
> 
>      54 unsigned int svcrdma_max_requests = RPCRDMA_MAX_REQUESTS;
>      
>     143 #define RPCRDMA_SQ_DEPTH_MULT   8
>     145 #define RPCRDMA_MAX_REQUESTS    16
> 
> A brief experiment shows that devattr.max_qp_wr = 16384 on recent
> Mellanox cards, so the actual sc_sq_depth value will be ruled
> by svcrdma_max_requests, and be 128.
> 
> The worst case is a 4K page machine writing a 1MB reply to a READ rpc,
> which if I understand the code correctly uses a WR per page plus one
> for the reply header, or 1024/4+1 = 257 WRs.  Now imagine all the nfsd
> threads(*) trying to do that to the same QP.  SGI runs with 128 threads.

Fair enough big data fills the SQ. 

> 
> In this scenario, every call to svc_rdma_send() stands a good chance
> of blocking.  The wait is not interruptible and has no timeout.  If
> the client's HCA has a conniption under this load, the server will
> have some large fraction of the nfsds unkillably blocked until the
> server's HCA gives up and reports an error (I presume it does this?).

I consider the client's hardware failing as outside the common
optimization case. But I think your argument still holds for big data.

> 
> 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. 

>  * I would expect there to be a client-side limit on the number of
>    outstanding READs so this doesn't normally happen, but part of the
>    reason for knfsd's wspace management is to avoid DoS attacks from
>    malicious or broken clients.
> 
> > There's actually a stat kept by the transport that counts the number of
> > times it waits.
> 
> Ah, that would be rdma_stat_sq_starve?  It's been added since the
> November code drop so I hadn't noticed it.  BTW is there a reason
> for emitting statistics to userspace as sysctls?

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.

> 
> > There is a place that a wait is done in the "normal" case and that's for
> > the completion of an RDMA_READ in the process of gathering the data for
> > and RPC on receive. That wait happens _every_ time.
> 
> Yes indeed, and this is the "worse" I was referring to.  I have
> a crash dump in which 122 of the 128 nfsd threads are doing this:
>  0 schedule+0x249c [0xa00000010052799c]
>  1 schedule_timeout+0x1ac [0xa00000010052958c]
>  2 rdma_read_xdr+0xf2c [0xa0000002216bcd8c]
>  3 svc_rdma_recvfrom+0x1e5c [0xa0000002216bed7c]
>  4 svc_recv+0xc8c [0xa00000022169f0ac]
>  5 nfsd+0x1ec [0xa000000221d7504c]
>  6 __svc_create_thread_tramp+0x30c [0xa0000002216976ac]
>  7 kernel_thread_helper+0xcc [0xa00000010001290c]
>  8 start_kernel_thread+0x1c [0xa0000001000094bc]
>                    
>              
>                                                                    
> That doesn't leave a lot of threads to do real work, like waiting for
> the filesystem.  

Hmm. I'd quibble with your example, but point taken. Other threads could
be doing work on another mount point for another client. 

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. 

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. 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?

> 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.

> 
> Greg.
> -- 
> Greg Banks, R&D Software Engineer, SGI Australian Software Group.
> Apparently, I'm Bedevere.  Which MPHG character are you?
> I don't speak for SGI.


-------------------------------------------------------------------------
This SF.net email is sponsored by DB2 Express
Download DB2 Express C - the FREE version of DB2 express and take
control of your XML. No limits. Just data. Click to get it now.
http://sourceforge.net/powerbar/db2/
_______________________________________________
NFS maillist  -  NFS@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/nfs