netdev - Re: [RFC] fib_trie: flush improvement

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Date:	Wed, 02 Apr 2008 16:35:04 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Stephen Hemminger <shemminger@...tta.com>,
	Robert Olsson <Robert.Olsson@...a.slu.se>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [RFC] fib_trie: flush improvement

Eric Dumazet a écrit :
> Stephen Hemminger a écrit :
>> This is an attempt to fix the problem described in:
>>      http://bugzilla.kernel.org/show_bug.cgi?id=6648
>> I can reproduce this by loading lots and lots of routes and the taking
>> the interface down. This causes all entries in trie to be flushed, but
>> each leaf removal causes a rebalance of the trie. And since the removal
>> is depth first, it creates lots of needless work.
>>
>> Instead on flush, just walk the trie and prune as we go.
>> The implementation is for description only, it probably doesn't work 
>> yet.
>>
>>   
>
> I dont get it, since the bug reporter mentions with recent kernels :
>
> Fix inflate_threshold_root. Now=15 size=11 bits
>
> Is it what you get with your tests ?
>
> Pawel reports :
>
> cat /proc/net/fib_triestat
> Main: Aver depth: 2.26 Max depth: 6 Leaves: 235924
> Internal nodes: 57854 1: 31632 2: 11422 3: 8475 4: 3755 5: 1676 6: 893 
> 18: 1
>
> Pointers: 609760 Null ptrs: 315983 Total size: 16240 kB
>
> warning messages comes from rootnode that cannot be expanded, since it 
> hits MAX_ORDER (on a 32bit x86)
>
>
>
> (sizeof(struct tnode) + (sizeof(struct node *) << bits);) is rounded 
> to 4 << (bit + 1), ie 2 << 20
>
> For larger allocations Pawel has two choices :
>
> change MAX_ORDER from 11 to 13 or 14
> If this machine is a pure router, this change wont have performance 
> impact.
>
> Or (more difficult, but more appropriate for mainline) change 
> fib_trie.c to use vmalloc() for very big allocaions (for the root 
> only), and vfree()
>
> Since vfree() cannot be called from rcu callback, one has to setup a 
> struct work_struct helper.
>
Here is a patch (untested unfortunatly) to implement this.

[IPV4] fib_trie: root_tnode can benefit of vmalloc()

FIB_TRIE root node can be very large and currently hits MAX_ORDER limit.
It also wastes about 50% of allocated size, because of power of two 
rounding of tnode.

A switch to vmalloc() can improve FIB_TRIE performance by allowing root 
node to grow
past the alloc_pages() limit, while preserving memory.

Special care must be taken to free such zone, as rcu handler is not 
allowed to call vfree(),
we use a worker instead.

Signed-off-by: Eric Dumazet <dada1@...mosbay.com>



View attachment "trie_vmalloc.patch" of type "text/plain" (2326 bytes)