Re: Please revert [PATCH] user of the jiffies rounding patch: Slab

From: Christoph Lameter
Date: Sat Apr 28 2007 - 19:36:51 EST

On Sat, 28 Apr 2007, Arjan van de Ven wrote:

> Christoph Lameter wrote:
> > The slab reaper takes global locks. If one makes all cache reapers fire at
> > the same time as this patch does then there will be a lot of contention that
> > may result lots of interrupt holdoffs since some locks are taken
> > with interrupts disabled. The vm statistics counters are updated
> > and will content for global cachelines if this is done.
> > I'd suggest to use a staggered per cpu approach instead that runs multiple
> > per cpu timers at once. But every batch of these timers must be run at an
> > offset from each other. Not at the same time please.
> from kernel/timer.c
> 107
> 108 /*
> 109 * We don't want all cpus firing their timers at once hitting the
> 110 * same lock or cachelines, so we skew each extra cpu with an
> extra
> 111 * 3 jiffies. This 3 jiffies came originally from the mm/ code
> which
> 112 * already did this.
> 113 * The skew is done by adding 3*cpunr, then round, then subtract
> this
> 114 * extra offset again.
> 115 */
> 116 j += cpu * 3;
> isn't 3 jiffies distance per cpu enough for this already ??
> That's what it used to be before as well...

Hmmm... Yes but that looks like it comes from a different function. Slab

__round_jiffies_relative(HZ, cpu))

And its description says:

* __round_jiffies_relative - function to round jiffies to a full second
* @j: the time in (relative) jiffies that should be rounded
* @cpu: the processor number on which the timeout will happen
* __round_jiffies_relative() rounds a time delta in the future (in jiffies)
* up or down to (approximately) full seconds. This is useful for timers
* for which the exact time they fire does not matter too much, as long as
* they fire approximately every X seconds.
* By rounding these timers to whole seconds, all such timers will fire
* at the same time, rather than at various times spread out. The goal
* of this is to have the CPU wake up less, which saves power.
* The exact rounding is skewed for each processor to avoid all
* processors firing at the exact same time, which could lead
* to lock contention or spurious cache line bouncing.
* The return value is the rounded version of the @j parameter.

This is exactly the wrong thing to do.

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