Re: [PATCH] Making i/dhash_entries cmdline work as it use to.

[David Howells <dhowells@redhat.com>]

> Actualy, it doesnt look like we need MAX_SYS_HASH_TABLE_ORDER at all so
> I'm resending the patch which now limits the max size of a hash table to
> 1/16 total memory pages.  This would keep people from doing dangerous
> things when using the hash_entries.

That's an enormous limit. Consider the fact that you will have a multiplicity
of such hash tables, each potentially eating 1/16th of your total memory
(remember, the bootmem allocator's only real limit is how big a chunk of
memory it can allocate in one go).

Do you have numbers to show that committing an eighth of your memory (8GB if
you have 64GB - two hash tables at 4GB apiece) to hash tables is almost
certainly not worth it.

For instance, the dcache:

	(*) Assuming 64GB of memory, of which 1/16th allocated to the dcache
	    hash table and 1/16th allocated to the inode hash table.

	(*) dcache hash buckets are 16 bytes apiece on PPC64.

	(*) 4GB hash table gives you 256 Meg hash buckets.

		(gdb) p ((4ULL<<30)/16) / 1024 / 1024
		$1 = 256

	(*) The dentry struct on PPC64 is approx 256 bytes in size.

	(*) The remaining 56GB could provide 224 Meg dentries if allocated to
	    nothing else.

		(gdb) p (56ULL<<30) / 256 / 1024 / 1024
		$2 = 224

	(*) So you'd end up with more buckets in the hash table than you could
	    possibly allocate dentry structures.

David