linux-kernel - Re: [PATCH] Introduce jiffies_32 and related compare functions

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Date:	Tue, 12 Dec 2006 04:47:14 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	David Miller <davem@...emloft.net>
Cc:	akpm@...l.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Introduce jiffies_32 and related compare functions

David Miller a écrit :
> From: Eric Dumazet <dada1@...mosbay.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@...mosbay.com>

View attachment "inetpeer.patch" of type "text/plain" (771 bytes)