Re: [PATCH] Prezeroing V8

[Pavel Machek <pavel@ucw.cz>]

Hi!

> Changelog:
> - Drop clear_pages and the approach to zero pages of higher order
>   first
> - Zero a percentage of pages from all orders to avoid fragmentation
> 
> Adds management of ZEROED and NOT_ZEROED pages and a background daemon
> called scrubd. /proc/sys/vm/scrubd_load, /proc/sys/vm_scrubd_start and
> /proc/sys/vm_scrubd_stop control the scrub daemon. See Documentation/vm/
> scrubd.txt
> 
> In an SMP environment the scrub daemon is typically running on the most
> idle cpu. Thus a single threaded application running
> on one cpu may have the other cpu zeroing pages for it etc. The scrub
> daemon is hardly noticable and usually finishes zeroing quickly since
> most processors are optimized for linear memory filling.
> 
> Patch against 2.6.11.3-bk3

> +unsigned int sysctl_scrub_start = 100;	/* Min percentage of zeroed free pages per zone (~10% default) */
> +unsigned int sysctl_scrub_stop = 300;	/* Max percentage of zeroed free pages per zone (~30% default) */
> +unsigned int sysctl_scrub_load = 1;	/* Do not run scrubd if load > */


Perhaps that variable should be called sysctl_scrub*d*_load?

....
> +        while (avenrun[0] >= ((unsigned long)sysctl_scrub_load << FSHIFT))
> +		schedule_timeout(30*HZ);
> +
> +	for (i = 0; i < pgdat->nr_zones; i++)
> +		zero_zone(pgdat->node_zones +i);

tabs vs. spaces alert, but more importantly....
> --- /dev/null	1970-01-01 00:00:00.000000000 +0000
> +++ linux-2.6.11/Documentation/vm/scrubd.txt	2005-03-17 12:57:41.000000000 -0800
> @@ -0,0 +1,59 @@
> +The SCRUB Daemon
> +----------------
> +
> +The scrub daemon zeroes memory so that later requests for zeroed memory can
> +be satisifed without having to zero memory in a hot code path. The operations
> +of scrubd may be controlled through 3 variables in /proc/sys/vm:
> +
> +/proc/sys/vm/scrubd_load	default value	1
> +
> +	Scrubd is woken up if the system load is lower than this setting and
> +	the numer of unzeroed free pages drops below scrub_start*10 percent.
> +	The default setting of 1 insures that there will be no performance
> +	degradation in single user mode. In an SMP system a cpu is frequently
> +	idle despite the load being high. A setting of 9 or 99 may
> +	be useful then.

I don't think 1 guarantees no performance degradation. After all, it
is average load and scrubd might run at just the wrong times. Perhaps
it should default to 0?

								Pavel
-- 
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