vfs.read_min (was Re: svn commit: r250906 - stable/9/sys/kern)

[Mike Tancsa]

Hi Scott,
	This seems to significantly help for reading through large files (argus
/ netflow files in my case) on my zfs server.  Doing some quick tests
and setting the size to 2 makes a difference from the default.  In your
case, did you find some optimal settings ?  Are there some tradeoffs /
caveats to setting this value to a non default value ?

	---Mike

On 5/22/2013 4:44 AM, Scott Long wrote:
> Author: scottl
> Date: Wed May 22 08:44:21 2013
> New Revision: 250906
> URL: http://svnweb.freebsd.org/changeset/base/250906
> 
> Log:
>   MFC r250327
>   
>   Add a sysctl vfs.read_min to complement the exiting vfs.read_max.  It
>   defaults to 1, meaning that it's off.
>   
>   When read-ahead is enabled on a file, the vfs cluster code deliberately
>   breaks a read into 2 I/O transactions; one to satisfy the actual read,
>   and one to perform read-ahead.  This makes sense in low-latency
>   circumstances, but often produces unbalanced i/o transactions that
>   penalize disks.  By setting vfs.read_min, we can tell the algorithm to
>   fetch a larger transaction that what we asked for, achieving the same
>   effect as the read-ahead but without the doubled, unbalanced transaction
>   and the slightly lower latency.  This significantly helps our workloads
>   with video streaming.
>   
>   Submitted by:   emax
>   Reviewed by:    kib
>   Obtained from:  Netflix
> 
> Modified:
>   stable/9/sys/kern/vfs_cluster.c
> Directory Properties:
>   stable/9/sys/   (props changed)
> 
> Modified: stable/9/sys/kern/vfs_cluster.c
> ==============================================================================
> --- stable/9/sys/kern/vfs_cluster.c	Wed May 22 07:52:41 2013	(r250905)
> +++ stable/9/sys/kern/vfs_cluster.c	Wed May 22 08:44:21 2013	(r250906)
> @@ -75,6 +75,10 @@ static int read_max = 64;
>  SYSCTL_INT(_vfs, OID_AUTO, read_max, CTLFLAG_RW, &read_max, 0,
>      "Cluster read-ahead max block count");
>  
> +static int read_min = 1;
> +SYSCTL_INT(_vfs, OID_AUTO, read_min, CTLFLAG_RW, &read_min, 0,
> +    "Cluster read min block count");
> +
>  /* Page expended to mark partially backed buffers */
>  extern vm_page_t	bogus_page;
>  
> @@ -169,6 +173,7 @@ cluster_read(vp, filesize, lblkno, size,
>  	} else {
>  		off_t firstread = bp->b_offset;
>  		int nblks;
> +		long minread;
>  
>  		KASSERT(bp->b_offset != NOOFFSET,
>  		    ("cluster_read: no buffer offset"));
> @@ -176,6 +181,13 @@ cluster_read(vp, filesize, lblkno, size,
>  		ncontig = 0;
>  
>  		/*
> +		 * Adjust totread if needed
> +		 */
> +		minread = read_min * size;
> +		if (minread > totread)
> +			totread = minread;
> +
> +		/*
>  		 * Compute the total number of blocks that we should read
>  		 * synchronously.
>  		 */
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> 
> 


-- 
-------------------
Mike Tancsa, tel +1 519 651 3400
Sentex Communications, mike at sentex.net
Providing Internet services since 1994 www.sentex.net
Cambridge, Ontario Canada   http://www.tancsa.com/