Messages in this thread | | | Date | Tue, 19 Feb 2008 08:35:36 +0100 | From | Eric Dumazet <> | Subject | Re: tbench regression in 2.6.25-rc1 |
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Zhang, Yanmin a écrit : > 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. >
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 was curious of the potential gain on your 16 cores (4x4) machine. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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