1. 30 Nov, 2005 1 commit
    • Mike Stroyan's avatar
      [IPV4] tcp/route: Another look at hash table sizes · 18955cfc
      Mike Stroyan authored
        The tcp_ehash hash table gets too big on systems with really big memory.
      It is worse on systems with pages larger than 4KB.  It wastes memory that
      could be better used.  It also makes the netstat command slow because reading
      /proc/net/tcp and /proc/net/tcp6 needs to go through the full hash table.
      
        The default value should not be larger for larger page sizes.  It seems
      that the effect of page size is an unintended error dating back a long
      time.  I also wonder if the default value really should be a larger
      fraction of memory for systems with more memory.  While systems with
      really big ram can afford more space for hash tables, it is not clear to
      me that they benefit from increasing the allocation ratio for this table.
      
        The amount of memory allocated is determined by net/ipv4/tcp.c:tcp_init and
      mm/page_alloc.c:alloc_large_system_hash.
      
      tcp_init calls alloc_large_system_hash passing parameters-
          bucketsize=sizeof(struct tcp_ehash_bucket)
          numentries=thash_entries
          scale=(num_physpages >= 128 * 1024) ? (25-PAGE_SHIFT) : (27-PAGE_SHIFT)
          limit=0
      
      On i386, PAGE_SHIFT is 12 for a page size of 4K
      On ia64, PAGE_SHIFT defaults to 14 for a page size of 16K
      
      The num_physpages test above makes the allocation take a larger fraction
      of the total memory on systems with larger memory.  The threshold size
      for a i386 system is 512MB.  For an ia64 system with 16KB pages the
      threshold is 2GB.
      
      For smaller memory systems-
      On i386, scale = (27 - 12) = 15
      On ia64, scale = (27 - 14) = 13
      For larger memory systems-
      On i386, scale = (25 - 12) = 13
      On ia64, scale = (25 - 14) = 11
      
        For the rest of this discussion, I'll just track the larger memory case.
      
        The default behavior has numentries=thash_entries=0, so the allocated
      size is determined by either scale or by the default limit of 1/16 of
      total memory.
      
      In alloc_large_system_hash-
      |	numentries = (flags & HASH_HIGHMEM) ? nr_all_pages : nr_kernel_pages;
      |	numentries += (1UL << (20 - PAGE_SHIFT)) - 1;
      |	numentries >>= 20 - PAGE_SHIFT;
      |	numentries <<= 20 - PAGE_SHIFT;
      
        At this point, numentries is pages for all of memory, rounded up to the
      nearest megabyte boundary.
      
      |	/* limit to 1 bucket per 2^scale bytes of low memory */
      |	if (scale > PAGE_SHIFT)
      |		numentries >>= (scale - PAGE_SHIFT);
      |	else
      |		numentries <<= (PAGE_SHIFT - scale);
      
      On i386, numentries >>= (13 - 12), so numentries is 1/8196 of
      bytes of total memory.
      On ia64, numentries <<= (14 - 11), so numentries is 1/2048 of
      bytes of total memory.
      
      |        log2qty = long_log2(numentries);
      |
      |        do {
      |                size = bucketsize << log2qty;
      
      bucketsize is 16, so size is 16 times numentries, rounded
      down to a power of two.
      
      On i386, size is 1/512 of bytes of total memory.
      On ia64, size is 1/128 of bytes of total memory.
      
      For smaller systems the results are
      On i386, size is 1/2048 of bytes of total memory.
      On ia64, size is 1/512 of bytes of total memory.
      
        The large page effect can be removed by just replacing
      the use of PAGE_SHIFT with a constant of 12 in the calls to
      alloc_large_system_hash.  That makes them more like the other uses of
      that function from fs/inode.c and fs/dcache.c
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
      18955cfc
  2. 29 Nov, 2005 32 commits
  3. 28 Nov, 2005 7 commits