Date: Sun, 17 Jan 1999 21:14:25 -0800 (PST) From: Matthew Dillon <dillon@apollo.backplane.com> To: "John S. Dyson" <root@dyson.iquest.net>, dg@root.com, jkh@FreeBSD.ORG, hackers@FreeBSD.ORG Subject: Found problem w/ Paging performance over NFS Message-ID: <199901180514.VAA57485@apollo.backplane.com>
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This is pretty cool - I figured out why the paging performance is so
horrible over NFS but good to local disk.
It turns out that the NFS subsystem looks at B_ASYNC in the bp. This
flag is not set by the paging code, so the NFS writes requested by the
pageout daemon were being done synchronously.
Setting B_ASYNC makes the paging go a whole lot faster, but creates
other problems relating to pbgetvp() and pbrelvp() calls - these calls
associate a vnode with a bp without linking the bp into the vnode or
messing with the vnode's reference count.
The problem is that NFS ( and possibly other subsystems), assumes that
it can call reassignbuf() on any bp when, in fact, it can't. bp's
handed to them from the paging system ( or any other subsystem that uses
pbgetvp() ) cannot be reassigned using reassignbuf(). nfs_doio() hits
this sometimes when you ^C a program.
In anycase, I fixed things up to formally separate bp's that use
pbgetvp() and bp's that do not, put in some asserts to enforce it,
and fixed the code in nfs_doio() that was messing things up. The
B_PAGING flag is now associated specifically with 'bp's that quickly
assign b_vp without messing with links or reference counts'.
I've also upgraded my LAN to 100BaseTX to test it. With 4 nfsiod's
running, I am getting excellent paging performance to NFS swap - 1.5 to
2.5 megabytes a second on pageout instead of 250 KBytes/sec. The diskless
workstation now pages out as well as a normal server!
-Matt
Matthew Dillon
<dillon@backplane.com>
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