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Date: Wed, 20 Feb 2008 15:04:41 +0800
From: "Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
To: Eric Dumazet <dada1@...mosbay.com>
Cc: Valdis.Kletnieks@...edu, David Miller <davem@...emloft.net>,
herbert@...dor.apana.org.au, linux-kernel@...r.kernel.org,
netdev@...r.kernel.org
Subject: Re: tbench regression in 2.6.25-rc1
On Tue, 2008-02-19 at 08:40 +0100, Eric Dumazet wrote:
> Zhang, Yanmin a écrit :
> > On Mon, 2008-02-18 at 12:33 -0500, Valdis.Kletnieks@...edu wrote:
> >> On Mon, 18 Feb 2008 16:12:38 +0800, "Zhang, Yanmin" said:
> >>
> >>> 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@...el.com>
> >> Could you add a comment someplace that says "refcnt wants to be on a different
> >> cache line from input/output/ops or performance tanks badly", to warn some
> >> future kernel hacker who starts adding new fields to the structure?
> > Ok. Below is the new patch.
> >
> > 1) Move tclassid under ops in case CONFIG_NET_CLS_ROUTE=y. So sizeof(dst_entry)=200
> > no matter if CONFIG_NET_CLS_ROUTE=y/n. I tested many patches on my 16-core tigerton by
> > moving tclassid to different place. It looks like tclassid could also have impact on
> > performance.
> > If moving tclassid before metrics, or just don't move tclassid, the performance isn't
> > good. So I move it behind metrics.
> >
> > 2) Add comments before __refcnt.
> >
> > If CONFIG_NET_CLS_ROUTE=y, the result with below patch is about 18% better than
> > the one without the patch.
> >
> > If CONFIG_NET_CLS_ROUTE=n, the result with below patch is about 30% better than
> > the one without the patch.
> >
> > Signed-off-by: Zhang Yanmin <yanmin.zhang@...el.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 2008-02-22 12:52:19.000000000 +0800
> > @@ -52,15 +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 */
> >
> > -#ifdef CONFIG_NET_CLS_ROUTE
> > - __u32 tclassid;
> > -#endif
> > + struct dst_entry *path;
> >
> > struct neighbour *neighbour;
> > struct hh_cache *hh;
> > @@ -70,10 +65,20 @@ struct dst_entry
> > int (*output)(struct sk_buff*);
> >
> > struct dst_ops *ops;
> > -
> > - unsigned long lastuse;
> > +
> > + u32 metrics[RTAX_MAX];
> > +
> > +#ifdef CONFIG_NET_CLS_ROUTE
> > + __u32 tclassid;
> > +#endif
> > +
> > + /*
> > + * __refcnt wants to be on a different cache line from
> > + * input/output/ops or performance tanks badly
> > + */
> > atomic_t __refcnt; /* client references */
> > int __use;
> > + unsigned long lastuse;
> > union {
> > struct dst_entry *next;
> > struct rtable *rt_next;
> >
> >
> >
>
> I prefer this patch, but unfortunatly your perf numbers are for 64 bits kernels.
>
> Could you please test now with 32 bits one ?
I tested it with 32bit 2.6.25-rc1 on 8-core stoakley. The result almost has no difference
between pure kernel and patched kernel.
New update: On 8-core stoakley, the regression becomes 2~3% with kernel 2.6.25-rc2. On
tigerton, the regression is still 30% with 2.6.25-rc2. On Tulsa( 8 cores+hyperthreading),
the regression is still 4% with 2.6.25-rc2.
With my patch, on tigerton, almost all regression disappears. On tulsa, only about 2%
regression disappears.
So this issue is triggerred with multiple-cpu. Perhaps process scheduler is another
factor causing the issue to happen, but it's very hard to change scheduler.
Eric,
I tested your new patch in function loopback_xmit. It has no improvement, while it doesn't
introduce new issues. As you tested it on dual-core machine and got improvement, how about
merging your patch with mine?
-yanmin
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