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From: "John S. Dyson" <root@dyson.iquest.net>
Message-Id: <199901190120.UAA27272@dyson.iquest.net>
Subject: Re: Found problem w/ Paging performance over NFS
In-Reply-To: <199901190001.QAA82498@apollo.backplane.com> from Matthew Dillon at "Jan 18, 99 04:01:52 pm"
To: dillon@apollo.backplane.com (Matthew Dillon)
Date: Mon, 18 Jan 1999 20:20:06 -0500 (EST)
Cc: dg@root.com, jkh@FreeBSD.ORG, hackers@FreeBSD.ORG
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>     Here's another piece of code to check:
> 
>     in kern/vfs_bio.c, allocbuf()
> 
> 	    } else if (m->flags & PG_BUSY) {
> 		    s = splvm();
> 		    if (m->flags & PG_BUSY) {
> 			    vm_page_flag_set(m, PG_WANTED);
> 			    tsleep(m, PVM, "pgtblk", 0);
> 		    }  
> 		    splx(s);
> 		    goto doretry;
> 	    } else {
> 		    ...
> 		    if (tinc > (newbsize - toff))
> 			    tinc = newbsize - toff;
> 		    if (bp->b_flags & B_CACHE)
> 			    vfs_buf_set_valid(bp, off, toff, tinc, m);
> 		    vm_page_flag_clear(m, PG_ZERO);
> 		    vm_page_wire(m);
> 	    }
> 
>     Shouldn't those conditionals be 'm->busy || (m->flags & PG_BUSY)' instead
>     of just testing against PG_BUSY?  A pageout operation will set m->busy
>     without setting PG_BUSY.
> 
The existing code is okay.  Pageouts don't need to set PG_BUSY.  Also, *generally*
during buffer cache read write operations, the PG_BUSY flag doesn't need to be set
during the *entire* operation.  Think filesystems with block sizes < PAGE_SIZE
as an example.

PG_BUSY is quite severe, and not always needed.

John

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