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Date:	Thu, 27 Feb 2025 09:49:40 +0100
To:	cygwin AT cygwin DOT com
Subject:	Re: rm -Rfv on UNC path horribly slow in Cygwin
	3.6.0-0.404.g323729f654ae.x86_64 ...
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From:	Corinna Vinschen via Cygwin <cygwin AT cygwin DOT com>
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On Feb 27 05:58, Roland Mainz via Cygwin wrote:
> On Wed, Feb 26, 2025 at 9:26 PM Corinna Vinschen via Cygwin
> <cygwin AT cygwin DOT com> wrote:
> > On Feb 26 20:16, Roland Mainz via Cygwin wrote:
> > > Something is wrong with UNC paths in Cygwin 3.6 build
> > > 3.6.0-0.404.g323729f654ae.x86_64 and ms-nfs41-client 2025-02-21:
> > >
> > > Example:
> > > ---- snip ----
> > > [...]
> > > ---- snip ----
> > >
> > > Each line from rm -Rfv now needs around 3-5 seconds. Switching to CWD
> > > //derfwnb4966_ipv4 AT 2049/nfs4/bigdisk/builds/bash_build1 (e.g. non-FQDN
> > > for share host) does not fix the problem. Going to
> > > /cygdrive/l/builds/bash_build1 and the rm -Rfv deletes around 70-90
> > > files per second.
> > > Last tested version was Cygwin commit
> > > #4bcc6adec765ee8354bfc9e6c060c9d1c0c7bf46 (and same ms-nfs41-client
> > > 2025-02-21 release), rm -Rfv performance there was normal (same as on
> > > /cygdrive/l/...).
> >
> > I can't reproduce this on MS NFS and SMB.
> >
> > You should try this with the latest 406 build.
> 
> Done, works perfectly - see below, the issue was something different...

I'm glad to read that.

> > If this doesn't fix
> > it, I need to know which test release introduced this problem.
> 
> It turns out this is something very NFSv4.x-specific: Delegations!
> 
> A NFSv4.x server can grant a client a "read" or "write" delegation. A
> client can cache reads or read/writes locally if the server grants
> that delegation. If another client touches the file for which the
> server has granted a delegation to another client, the delegation must
> be recalled first and the caller gets a |NFS4ERR_DELAY|, and has to
> re-try after a delay.
> 
> And this happens in this case, because the mounts I used for testing
> are (intentionally [1]) treated as separate clients.
> 
> [1]=First it's my test setup, and secondly the ms-nfs41-client is
> highly threaded to deal with the async nature of the Windows kernel
> and NFSv4.x's nature of being able to handle requests in parallel, the
> only limiting factor being TCP and the RPC implementations's locking.
> 
> Wireshark documented this nicely:
> ---- snip ----
> [...]
> ---- snip ----
> 
> So everything is working as expected, but I really need a FSCTL_* to
> flush all delegations for accurate performance benchmarking.
> 
> Does the Win32 have a "flush cache"-|FSCTL_*|/|IOCTL_*| ?

I don't know any FSCTL/IOCTL for that.

I'm only aware of the FlushFileBuffers() call for single files, as well
as the SetSystemFileCacheSize() function as a way to flush the entire
filesystem cache.
There's also a way to flush the cache for a single filesystem, but
I think this works only for local filesystems:
https://stackoverflow.com/questions/7405868/how-can-i-invalidate-the-file-system-cache


Corinna

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