Re: [PATCH 3/3] nfnetlink_queue: use hash table to speed up entry finding.

[Changli Gao <xiaosuo@gmail.com>]

On Tue, Apr 13, 2010 at 6:09 PM, Patrick McHardy <kaber@trash.net> wrote:
> Changli Gao wrote:
>> use hash table to speed up entry finding.
>>
>> If verdicts aren't received in order, list isn't efficient, and hash
>> table is better.
>
> Any what is the advantage of using flex arrays compared to a simple
> open code hash table?
>

From Documentation/flexible-arrays.txt

Large contiguous memory allocations can be unreliable in the Linux kernel.
Kernel programmers will sometimes respond to this problem by allocating
pages with vmalloc().  This solution not ideal, though.  On 32-bit systems,
memory from vmalloc() must be mapped into a relatively small address space;
it's easy to run out.  On SMP systems, the page table changes required by
vmalloc() allocations can require expensive cross-processor interrupts on
all CPUs.  And, on all systems, use of space in the vmalloc() range
increases pressure on the translation lookaside buffer (TLB), reducing the
performance of the system.

User may create lots of queues, which have large hash tables. Flex
array has better scalability than vmalloc() and kmalloc().

-- 
Regards，
Changli Gao(xiaosuo@gmail.com)
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