David Miller a écrit :
From: Eric Dumazet <[email protected]>
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 <[email protected]>
--- 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);
}
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