Date: Sun, 12 Jun 2005 06:30:49 -0400 (EDT) From: Daniel Eischen <deischen@freebsd.org> To: Alfred Perlstein <alfred@freebsd.org> Cc: Brian Fundakowski Feldman <green@freebsd.org>, src-committers@freebsd.org, cvs-all@freebsd.org, cvs-src@freebsd.org Subject: Re: cvs commit: src/sys/nfsclient nfs_bio.c nfs_vfsops.c nfsargs.h nfsmount.h src/sys/sys buf.h bufobj.h src/sys/kern vfs_bio.c Message-ID: <Pine.GSO.4.43.0506120610030.18217-100000@sea.ntplx.net> In-Reply-To: <20050612100708.GK17867@elvis.mu.org>
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On Sun, 12 Jun 2005, Alfred Perlstein wrote: > * Brian Fundakowski Feldman <green@freebsd.org> [050612 01:26] wrote: > > On Sun, Jun 12, 2005 at 01:08:33AM -0700, Alfred Perlstein wrote: > > > > > > Seriously, have you tested what happens to a libc_r app that > > > opens an nfs file F_SYNC? My guess is that it's not pretty. > > > > This code path is related to O_NONBLOCK, not O_FSYNC. O_FSYNC is > > synonymous with the slow fallback path that large transactional block > > now takes, rather than deadlocking. O_NONBLOCK really means that > > whatever they do, they are required to check for EAGAIN. > > To make it perfectly clear. > > If an application linked against libc_r opens a file with O_FSYNC. > Libc_r will set O_NONBLOCK (it does so for each open(2)) > A write on that descriptor will return EAGAIN (to libc_r) > Libc_r will then attempt to select(2) on this decriptor, which > will return "ready" (as do all select(2)'s on disk files) > > The question is: > > Will Libc_r then busy spin? Yes, for the most part. > If so, how many other apps might get screwed just sometimes (over > nfs) because only _half_ of this "solution" is implemented? > > Or is my thinking on this wrong? I think I agree with Alfred. If select() returns ready, then you should be able to write some part of your buffer. Also, anyone using a file descriptor in non-blocking mode should expect short writes and loop until the entire buffer has been written. >From my understanding, disk I/O has always returned ready, then blocked in the kernel if necessary. I think that if we are going to start honoring non-blocking mode for disk I/O (or NFS, whatever), it should be done fully. That means you allow short writes when select() returns ready, and select() doesn't return ready if no data can be written. -- DE
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