netdev - [PATCH] udp: extend hash tables to 256 slots

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Date:	Wed, 07 Oct 2009 06:43:31 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"David S. Miller" <davem@...emloft.net>
CC:	Rick Jones <rick.jones2@...com>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: [PATCH] udp: extend hash tables to 256 slots

Eric Dumazet a écrit :
> I was going to setup a bench lab, with a typical RTP mediaserver, with say
> 4000 UDP sockets, 2000 sockets exchanging 50 G.711 Alaw/ulaw
> messages per second tx and rx. (Total : 100.000 packets per second each way)
> 

Hmm, it seems we'll have too many sockets per udp hash chain unfortunatly for this
workload to show any improvement.

(~32 sockets per chain : average of 16 misses to lookup the target socket.)

David, I believe UDP_HTABLE_SIZE never changed from its initial value of 128,
defined 15 years ago. Could we bump it to 256 ?

(back in 1995, SOCK_ARRAY_SIZE was 256)

(I'll probably use 1024 value for my tests)

[PATCH] udp: extend hash tables to 256 slots

UDP_HTABLE_SIZE was initialy defined to 128, which is a bit small for several setups.
4000 active sockets -> 32 sockets per chain in average.

Doubling hash table size has a memory cost of 128 (pointers + spinlocks) for UDP,
same for UDPLite, this should be OK.

It reduces the size of bitmap used in udp_lib_get_port() and speedup port allocation.
#define PORTS_PER_CHAIN (65536 / UDP_HTABLE_SIZE) -> 256 bits instead of 512 bits

Use CONFIG_BASE_SMALL to keep hash tables small for small machines.

Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
---
diff --git a/include/linux/udp.h b/include/linux/udp.h
index 0cf5c4c..8aaa151 100644
--- a/include/linux/udp.h
+++ b/include/linux/udp.h
@@ -45,7 +45,7 @@ static inline struct udphdr *udp_hdr(const struct sk_buff *skb)
 	return (struct udphdr *)skb_transport_header(skb);
 }

-#define UDP_HTABLE_SIZE		128
+#define UDP_HTABLE_SIZE		(CONFIG_BASE_SMALL ? 128 : 256)

 static inline int udp_hashfn(struct net *net, const unsigned num)
 {
--
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