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SubjectRe: tbench regression in 2.6.25-rc1
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On Mon, 2008-02-18 at 11:11 +0100, Eric Dumazet wrote:
> On Mon, 18 Feb 2008 16:12:38 +0800
> "Zhang, Yanmin" <yanmin_zhang@linux.intel.com> wrote:
>
> > On Fri, 2008-02-15 at 15:22 -0800, David Miller wrote:
> > > From: Eric Dumazet <dada1@cosmosbay.com>
> > > Date: Fri, 15 Feb 2008 15:21:48 +0100
> > >
> > > > On linux-2.6.25-rc1 x86_64 :
> > > >
> > > > offsetof(struct dst_entry, lastuse)=0xb0
> > > > offsetof(struct dst_entry, __refcnt)=0xb8
> > > > offsetof(struct dst_entry, __use)=0xbc
> > > > offsetof(struct dst_entry, next)=0xc0
> > > >
> > > > So it should be optimal... I dont know why tbench prefers __refcnt being
> > > > on 0xc0, since in this case lastuse will be on a different cache line...
> > > >
> > > > Each incoming IP packet will need to change lastuse, __refcnt and __use,
> > > > so keeping them in the same cache line is a win.
> > > >
> > > > I suspect then that even this patch could help tbench, since it avoids
> > > > writing lastuse...
> > >
> > > I think your suspicions are right, and even moreso
> > > it helps to keep __refcnt out of the same cache line
> > > as input/output/ops which are read-almost-entirely :-
> > I think you are right. The issue is these three variables sharing the same cache line
> > with input/output/ops.
> >
> > > )
> > >
> > > I haven't done an exhaustive analysis, but it seems that
> > > the write traffic to lastuse and __refcnt are about the
> > > same. However if we find that __refcnt gets hit more
> > > than lastuse in this workload, it explains the regression.
> > I also think __refcnt is the key. I did a new testing by adding 2 unsigned long
> > pading before lastuse, so the 3 members are moved to next cache line. The performance is
> > recovered.
> >
> > How about below patch? Almost all performance is recovered with the new patch.
> >
> > Signed-off-by: Zhang Yanmin <yanmin.zhang@intel.com>
> >
> > ---
> >
> > --- linux-2.6.25-rc1/include/net/dst.h 2008-02-21 14:33:43.000000000 +0800
> > +++ linux-2.6.25-rc1_work/include/net/dst.h 2008-02-21 14:36:22.000000000 +0800
> > @@ -52,11 +52,10 @@ struct dst_entry
> > unsigned short header_len; /* more space at head required */
> > unsigned short trailer_len; /* space to reserve at tail */
> >
> > - u32 metrics[RTAX_MAX];
> > - struct dst_entry *path;
> > -
> > - unsigned long rate_last; /* rate limiting for ICMP */
> > unsigned int rate_tokens;
> > + unsigned long rate_last; /* rate limiting for ICMP */
> > +
> > + struct dst_entry *path;
> >
> > #ifdef CONFIG_NET_CLS_ROUTE
> > __u32 tclassid;
> > @@ -70,10 +69,12 @@ struct dst_entry
> > int (*output)(struct sk_buff*);
> >
> > struct dst_ops *ops;
> > -
> > - unsigned long lastuse;
> > +
> > + u32 metrics[RTAX_MAX];
> > +
> > atomic_t __refcnt; /* client references */
> > int __use;
> > + unsigned long lastuse;
> > union {
> > struct dst_entry *next;
> > struct rtable *rt_next;
> >
> >
>
> Well, after this patch, we grow dst_entry by 8 bytes :
With my .config, it doesn't grow. Perhaps because of CONFIG_NET_CLS_ROUTE, I don't
enable it. I will move tclassid under ops.

>
> sizeof(struct dst_entry)=0xd0
> offsetof(struct dst_entry, input)=0x68
> offsetof(struct dst_entry, output)=0x70
> offsetof(struct dst_entry, __refcnt)=0xb4
> offsetof(struct dst_entry, lastuse)=0xc0
> offsetof(struct dst_entry, __use)=0xb8
> sizeof(struct rtable)=0x140
>
>
> So we dirty two cache lines instead of one, unless your cpu have 128 bytes cache lines ?
>
> I am quite suprised that my patch to not change lastuse if already set to jiffies changes nothing...
>
> If you have some time, could you also test this (unrelated) patch ?
>
> We can avoid dirty all the time a cache line of loopback device.
>
> diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
> index f2a6e71..0a4186a 100644
> --- a/drivers/net/loopback.c
> +++ b/drivers/net/loopback.c
> @@ -150,7 +150,10 @@ static int loopback_xmit(struct sk_buff *skb, struct net_device *dev)
> return 0;
> }
> #endif
> - dev->last_rx = jiffies;
> +#ifdef CONFIG_SMP
> + if (dev->last_rx != jiffies)
> +#endif
> + dev->last_rx = jiffies;
>
> /* it's OK to use per_cpu_ptr() because BHs are off */
> pcpu_lstats = netdev_priv(dev);
>
Although I didn't test it, I don't think it's ok. The key is __refcnt shares the same
cache line with ops/input/output.

-yanmin




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