Re: [PATCH] limit rt cache size

[David Miller <davem@davemloft.net>]

From: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Date: Mon, 7 Aug 2006 20:48:42 +0400

> The patch looks OK. But I am not sure too.
> 
> To be honest, I do not understand the sense of HASH_HIGHMEM flag.
> At the first sight, hash table eats low memory, objects hashed in this table
> also eat low memory. Why is its size calculated from total memory?
> But taking into account that this flag is used only by tcp.c and route.c,
> both of which feed on low memory, I miss something important.
> 
> Let's ask people on netdev.

Is it not so hard to check history of the change to see where these
things come from?  :-) If we study the output of command:

	git whatchanged net/core/route.c

we quickly discover this GIT commit:

424c4b70cc4ff3930ee36a2ef7b204e4d704fd26

[IPV4]: Use the fancy alloc_large_system_hash() function for route hash table

- rt hash table allocated using alloc_large_system_hash() function.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

And it is clear that old code used num_physpages, which counts low
memory only.  This shows clearly that Eric's usage of the HASH_HIGHMEM
flag here is erroneous.  So we should remove it.

Look!  This thing even uses num_physpages in current code to compute
the "scale" argument to alloc_large_system_hash() :)))

> What's about routing cache size, it looks like it is another bug.
> route.c should not force rt_max_size = 16*rt_hash_size.
> I think it should consult available memory and to limit rt_max_size
> to some reasonable value, even if hash size is too high.

Sure.  This current setting of 16*rt_hash_size is meant to
try to limit hash chain lengths I guess.  2.4.x does the same
thing.  Note also that by basing it upon number of routing cache
hash chains, it is effectively consulting available memory.
This is why when hash table sizing is crap so it rt_max_size
calculation.  Fix one and you fix them both :)

Once the HASH_HIGHMEM flag is removed, assuming system has > 128K of
memory, what we get is:

	hash_chains = lowmem / 128K
	rt_max_size = ((lowmem / 128K) * 16) == lowmem / 8K

So we allow one routing cache entry for each 8K of lowmem we have :)

So for now it is probably sufficient to just get rid of the
HASH_HIGHMEM flag here.  Later we can try changing this multiplier
of "16" to something like "8" or even "4".