Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1236886

Re: [lkp] [nfsd] 4aac1bf05b: -2.9% fsmark.files_per_sec

Show all headers | View raw

On Thu, Oct 01, 2015 at 09:17:42AM +1000, Dave Chinner wrote:
> Inode pointers have no entropy in the lower 9-10 bits because of
> their size, and being allocated from a slab they are all going to
> have the same set of values in the next 3-4 bits (i.e. offset into
> the slab page which is defined by sizeof(inode)).  Pointers also
> have very similar upper bits, too, because they are all in kernel
> memory.
> 
> hash_64 trys to fold all the entropy from the lower bits into into
> the upper bits and then takes the result from the upper bits. Hence
> if there is no entropy in either the lower or upper bits to start
> with, then the hash may not end up with much entropy in it at all...

So we have something hash_ptr() that turns out to be terrible at hashing
pointers?  Argh.

(I understand you're saying this isn't necessarily the case for all
pointers, but inode pointers on their own seem likely to be a common
case, and there must be many more that are similar.)

--b.

> 
> FWIW, see fs/inode.c::hash() to see how the fs code hashes inode
> numbers (called from insert_inode_hash()). It's very different
> because because inode numbers have the majority of their entropy in
> the lower bits and (usually) none in the upper bits...
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Back to linux.kernel | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Re: [lkp] [nfsd] 4aac1bf05b: -2.9% fsmark.files_per_sec Jeff Layton <jeff.layton@primarydata.com> - 2015-09-29 13:50 +0200
  Re: [lkp] [nfsd] 4aac1bf05b: -2.9% fsmark.files_per_sec "Huang\, Ying" <ying.huang@linux.intel.com> - 2015-09-30 01:30 +0200
    Re: [lkp] [nfsd] 4aac1bf05b: -2.9% fsmark.files_per_sec Jeff Layton <jeff.layton@primarydata.com> - 2015-09-30 02:10 +0200
      Re: [lkp] [nfsd] 4aac1bf05b: -2.9% fsmark.files_per_sec "Huang\, Ying" <ying.huang@linux.intel.com> - 2015-09-30 10:40 +0200
        Re: [lkp] [nfsd] 4aac1bf05b: -2.9% fsmark.files_per_sec Jeff Layton <jeff.layton@primarydata.com> - 2015-09-30 12:10 +0200
          Re: [lkp] [nfsd] 4aac1bf05b: -2.9% fsmark.files_per_sec Dave Chinner <david@fromorbit.com> - 2015-10-01 01:20 +0200
            Re: [lkp] [nfsd] 4aac1bf05b: -2.9% fsmark.files_per_sec "J. Bruce Fields" <bfields@fieldses.org> - 2015-10-01 03:00 +0200
            Re: [lkp] [nfsd] 4aac1bf05b: -2.9% fsmark.files_per_sec Jeff Layton <jeff.layton@primarydata.com> - 2015-10-01 13:40 +0200

csiph-web