netdev - Re: [PATCH net-next] bnx2x: reduce skb truesize by 50%

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Message-ID: <1320937526.307.0.camel@lb-tlvb-eilong.il.broadcom.com>
Date:	Thu, 10 Nov 2011 17:05:26 +0200
From:	"Eilon Greenstein" <eilong@...adcom.com>
To:	"Eric Dumazet" <eric.dumazet@...il.com>
cc:	"David Miller" <davem@...emloft.net>,
	"bhutchings@...arflare.com" <bhutchings@...arflare.com>,
	"pstaszewski@...are.pl" <pstaszewski@...are.pl>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] bnx2x: reduce skb truesize by 50%

On Wed, 2011-11-09 at 16:29 -0800, Eric Dumazet wrote:
> Le mercredi 09 novembre 2011 à 23:03 +0100, Eric Dumazet a écrit :
> 
> > BTW, on my bnx2x adapter, even small UDP frames use more than PAGE_SIZE
> > bytes :
> > 
> > skb->truesize=4352 len=26 (payload only)
> > 
> 
> > I wonder if we shouldnt increase SK_MEM_QUANTUM a bit to avoid
> > ping/pong...
> > 
> > -#define SK_MEM_QUANTUM ((int)PAGE_SIZE)
> > +#define SK_MEM_QUANTUM ((int)PAGE_SIZE * 2)
> > 
> 
> Following patch also helps a lot, even with only two cpus (one handling
> device interrupts, one running the application thread)
> 
> [PATCH net-next] bnx2x: reduce skb truesize by ~50%
> 
> bnx2x uses following formula to compute its rx_buf_sz :
> 
> dev->mtu + 2*L1_CACHE_BYTES + 14 + 8 + 8
> 
> Then core network adds NET_SKB_PAD and SKB_DATA_ALIGN(sizeof(struct
> skb_shared_info))
> 
> Final allocated size for skb head on x86_64 (L1_CACHE_BYTES = 64,
> MTU=1500) : 2112 bytes : SLUB/SLAB round this to 4096 bytes.
> 
> Since skb truesize is then bigger than SK_MEM_QUANTUM, we have lot of
> false sharing because of mem_reclaim in UDP stack.
> 
> One possible way to half truesize is to lower the need by 64 bytes (2112
> -> 2048 bytes)
> 
> This way, skb->truesize is lower than SK_MEM_QUANTUM and we get better
> performance.
> 
> (760.000 pps on a rx UDP monothread benchmark, instead of 720.000 pps)
> 
> 
> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
> CC: Eilon Greenstein <eilong@...adcom.com>
> ---
>  drivers/net/ethernet/broadcom/bnx2x/bnx2x.h |   11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/ethernet/broadcom/bnx2x/bnx2x.h b/drivers/net/ethernet/broadcom/bnx2x/bnx2x.h
> index aec7212..ebbdc55 100644
> --- a/drivers/net/ethernet/broadcom/bnx2x/bnx2x.h
> +++ b/drivers/net/ethernet/broadcom/bnx2x/bnx2x.h
> @@ -1185,9 +1185,14 @@ struct bnx2x {
>  #define ETH_MAX_PACKET_SIZE		1500
>  #define ETH_MAX_JUMBO_PACKET_SIZE	9600
>  
> -	/* Max supported alignment is 256 (8 shift) */
> -#define BNX2X_RX_ALIGN_SHIFT		((L1_CACHE_SHIFT < 8) ? \
> -					 L1_CACHE_SHIFT : 8)
> +/* Max supported alignment is 256 (8 shift)
> + * It should ideally be min(L1_CACHE_SHIFT, 8)
> + * Choosing 5 (32 bytes) permits to get skb heads of 2048 bytes
> + * instead of 4096 bytes.
> + * With SLUB/SLAB allocators, data will be cache line aligned anyway.
> + */
> +#define BNX2X_RX_ALIGN_SHIFT		5
> +

Hi Eric,

This can seriously hurt the PCI utilization. So in scenarios in which
the PCI is the bottle neck, you will see performance degradation. We are
looking at alternatives to reduce the allocation, but it is taking a
while. Please hold off with this patch.

Thanks,
Eilon


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