On Tue, 2008-02-19 at 08:35 +0100, Eric Dumazet wrote: > Zhang, Yanmin a �crit : > > On Mon, 2008-02-18 at 11:11 +0100, Eric Dumazet wrote: > >> On Mon, 18 Feb 2008 16:12:38 +0800 > >> "Zhang, Yanmin" <[EMAIL PROTECTED]> wrote: > >> > >>> On Fri, 2008-02-15 at 15:22 -0800, David Miller wrote: > >>>> From: Eric Dumazet <[EMAIL PROTECTED]> > >>>> 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 <[EMAIL PROTECTED]> > >>> > >>> --- > >>> > >>> --- 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. > > > > Note it was unrelated to struct dst, but dirtying of one cache line of > 'loopback netdevice' > > I tested it, and tbench result was better with this patch : 890 MB/s instead > of 870 MB/s on a bi dual core machine. I tested your new patch and it doesn't help tbench.
On my 8-core stoakley machine, the regression is only 5%, but it's 30% on 16-core tigerton. It looks like the scalability is poor. > > > I was curious of the potential gain on your 16 cores (4x4) machine. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html