Messages in this thread Patch in this message | | | Date | Tue, 12 Dec 2006 04:47:14 +0100 | From | Eric Dumazet <> | Subject | Re: [PATCH] Introduce jiffies_32 and related compare functions |
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David Miller a écrit : > From: Eric Dumazet <dada1@cosmosbay.com> > Date: Mon, 11 Dec 2006 23:58:06 +0100 > >> Some subsystems dont need more than 32bits timestamps. >> >> See for example net/ipv4/inetpeer.c and include/net/tcp.h : >> #define tcp_time_stamp ((__u32)(jiffies)) >> >> >> Because most timeouts should work with 'normal jiffies' that are 32bits on >> 32bits platforms, it makes sense to be able to use only 32bits to store them >> and not 64 bits, to save ram. >> >> This patch introduces jiffies_32, and related comparison functions >> time_after32(), time_before32(), time_after_eq32() and time_before_eq32(). >> >> I plan to use this infrastructure in network code for example (struct >> dst_entry comes to mind). > > The TCP case is because the protocol limits the size of > the timestamp we can store in the TCP Timestamp option. > > Otherwise we would use the full 64-bit jiffies timestamp, > in order to have a larger window of values which would not > overflow. > > Since there is no protocol limitation involved in cases > such as dst_entry, I think we should keep it at 64-bits > on 64-bit platforms to make the wrap-around window as > large as possible. > > I really don't see any reason to make these changes. Yes, > you'd save some space, but one of the chief advantages of > 64-bit is that we get larger jiffies value windows. If > that has zero value, as your intended changes imply, then > we shouldn't need the default 64-bit jiffies either, by > implication.
Well, just to have similar functions to manipulate jiffies.
We already know that using a 32bits dtime in struct inet_peer permited an object size of 64 bytes instead of 128 bytes (on 64bits platforms)
On one machine (running linux-2.6.16) I have :
# grep peer /proc/slabinfo inet_peer_cache 65972 86220 128 30 1 : tunables 120 60 8 : slabdata 2874 2874 262
(So the 128 bytes -> 64 bytes is going to save 1437 pages of memory)
# grep dst /proc/slabinfo ip_dst_cache 1765818 2077380 320 12 1 : tunables 54 27 8 : slabdata 173115 173115 39
(So saving 4*4 bytes per dst might save 32 MB of ram). I doubt being able to extend the expiration of a dst above 2^32 ticks (49 days if HZ=1000, 198 days if HZ=250) is worth the ram wastage.
Dont you prefer to be able to apply this patch for example ?
Signed-off-by: Eric Dumazet <dada1@cosmosbay.com> --- linux/net/ipv4/inetpeer.c.orig 2006-12-12 05:25:42.000000000 +0100 +++ linux-ed/net/ipv4/inetpeer.c 2006-12-12 05:29:22.000000000 +0100 @@ -338,8 +338,7 @@ static int cleanup_once(unsigned long tt spin_lock_bh(&inet_peer_unused_lock); p = inet_peer_unused_head; if (p != NULL) { - __u32 delta = (__u32)jiffies - p->dtime; - if (delta < ttl) { + if (time_after32(p->dtime + ttl, jiffies_32)) { /* Do not prune fresh entries. */ spin_unlock_bh(&inet_peer_unused_lock); return -1; @@ -466,7 +465,7 @@ void inet_putpeer(struct inet_peer *p) p->unused_next = NULL; *inet_peer_unused_tailp = p; inet_peer_unused_tailp = &p->unused_next; - p->dtime = (__u32)jiffies; + p->dtime = jiffies_32; } spin_unlock_bh(&inet_peer_unused_lock); } | |