Re: NFS client latencies

[Ingo Molnar]

* Trond Myklebust <trond.myklebust@xxxxxxxxxx> wrote:

> > the comment suggests that this is optimized for append writes (which is 
> > quite common, but by far not the only write workload) - but the 
> > worst-case behavior of this code is very bad. How about disabling this 
> > sorting altogether and benchmarking the result? Maybe it would get 
> > comparable coalescing (higher levels do coalesce after all), but wastly 
> > improved CPU utilization on the client side. (Note that the server 
> > itself will do sorting of any write IO anyway, if this is to hit any 
> > persistent storage - and if not then sorting so agressively on the 
> > client side makes little sense.)
> 
> No. Coalescing on the client makes tons of sense. The overhead of
> sending 8 RPC requests for 4k writes instead of sending 1 RPC request
> for a single 32k write is huge: among other things, you end up tying up
> 8 RPC slots on the client + 8 nfsd threads on the server instead of just
> one of each.

yes - coalescing a few pages makes sense, but linearly scanning 
thousands of entries is entirely pointless.

	Ingo
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