Commit Graph

15 Commits

Author SHA1 Message Date
Ed Schouten
2747eff128 Replace implementation of hsearch() by one that scales.
Traditionally the hcreate() function creates a hash table that uses
chaining, using a fixed user-provided size. The problem with this
approach is that this often either wastes memory (table too big) or
yields bad performance (table too small). For applications it may not
always be easy to estimate the right hash table size. A fixed number
only increases performance compared to a linked list by a constant
factor.

This problem can be solved easily by dynamically resizing the hash
table. If the size of the hash table is at least doubled, this has no
negative on the running time complexity. If a dynamically sized hash
table is used, we can also switch to using open addressing instead of
chaining, which has the advantage of just using a single allocation for
the entire table, instead of allocating many small objects.

Finally, a problem with the existing implementation is that its
deterministic algorithm for hashing makes it possible to come up with
fixed patterns to trigger an excessive number of collisions. We can
easily solve this by using FNV-1a as a hashing algorithm in combination
with a randomly generated offset basis.

Measurements have shown that this implementation is about 20-25% faster
than the existing implementation (even if the existing implementation is
given an excessive number of buckets). Though it allocates more memory
through malloc() than the old implementation (between 4-8 pointers per
used entry instead of 3), process memory use is similar to the old
implementation as if the estimated size was underestimated by a factor
10. This is due to the fact that malloc() needs to perform less
bookkeeping.

Reviewed by:	jilles, pfg
Obtained from:	https://github.com/NuxiNL/cloudlibc
Differential Revision:	https://reviews.freebsd.org/D4644
2015-12-27 07:50:11 +00:00
Ed Schouten
459d04a5ee Let tsearch()/tdelete() use an AVL tree.
The existing implementations of POSIX tsearch() and tdelete() don't
attempt to perform any balancing at all. Testing reveals that inserting
100k nodes into a tree sequentially takes approximately one minute on my
system.

Though most other BSDs also don't use any balanced tree internally, C
libraries like glibc and musl do provide better implementations. glibc
uses a red-black tree and musl uses an AVL tree.

Red-black trees have the advantage over AVL trees that they only require
O(1) rotations after insertion and deletion, but have the disadvantage
that the tree has a maximum depth of 2*log2(n) instead of 1.44*log2(n).
My take is that it's better to focus on having a lower maximum depth,
for the reason that in the case of tsearch() the invocation of the
comparator likely dominates the running time.

This change replaces the tsearch() and tdelete() functions by versions
that create an AVL tree. Compared to musl's implementation, this version
is different in two different ways:

- We don't keep track of heights; just balances. This is sufficient.
  This has the advantage that it reduces the number of nodes that are
  being accessed. Storing heights requires us to also access all of the
  siblings along the path.

- Don't use any recursion at all. We know that the tree cannot 2^64
  elements in size, so the height of the tree can never be larger than
  96. Use a 128-bit bitmask to keep track of the path that is computed.
  This allows us to iterate over the same path twice, meaning we can
  apply rotations from top to bottom.

Inserting 100k nodes into a tree now only takes 0.015 seconds. Insertion
seems to be twice as fast as glibc, whereas deletion has about the same
performance. Unlike glibc, it uses a fixed amount of memory.

I also experimented with both recursive and iterative bottom-up
implementations of the same algorithm. This iterative top-down version
performs similar to the recursive bottom-up version in terms of speed
and code size.

For some reason, the iterative bottom-up algorithm was actually 30%
faster for deletion, but has a quadratic memory complexity to keep track
of all the parent pointers.

Reviewed by:	jilles
Obtained from:	https://github.com/NuxiNL/cloudlibc
Differential Revision:	https://reviews.freebsd.org/D4412
2015-12-22 18:12:11 +00:00
Pedro F. Giffuni
9823a90c79 Add re-entrant versions of the hash functions based on the GNU api.
While testing this I found a conformance issue in hdestroy()
that will be fixed in a subsequent commit.

Obtained from:	NetBSD (hcreate.c, CVS Rev. 1.7)
2014-07-21 15:22:48 +00:00
Pedro F. Giffuni
1d717f206a Revert r268826:
The current ordering of this header is a feature as it
is more consistent with POSIX.
Also adding gratuitous newlines is not elegant.

Pointed out by:	bde
2014-07-18 16:21:15 +00:00
Pedro F. Giffuni
7bd26d4567 Minor sorting to match the NetBSD header
MFC after:	3 days
Obtained from:	NetBSD
2014-07-18 02:49:41 +00:00
Robert Drehmel
6c84d0b1a5 - Remove the lsearch() and lfind() functions and their manpage from
the compatibility library libcompat.
 - Add new implementations of lsearch() and lfind() which conform to
   IEEE Std 1003.1-2001 to libc.  Add a new manual page for them and
   add them to the makefile.
 - Add function prototypes for lsearch() and lfind() to the search.h
   header.
2002-10-16 14:29:23 +00:00
Robert Drehmel
e768c1be41 - Remove the old insque() and remque() functions and their manual
page from the compatibility library.
 - Add new implementations of insque() and remque() which conform to
   IEEE Std 1003.1-2001 to libc.  Add a new manual page for them and
   connect them to the build.
 - Add the prototypes of insque() and remque() to the search.h
   header.
2002-10-16 14:00:46 +00:00
Mike Barcroft
a23db95772 Fix various style(9) bugs:
o Source ID's in wrong location.
o Space used, instead of tab, after typedef.
o Unaligned function prototype for twalk().

Other changes:
o Add missing const qualifier in tfind().
o Add comment about missing functions.
2002-10-03 06:31:16 +00:00
Tim J. Robbins
58d38e2520 Style: One space between "restrict" qualifier and "*". 2002-09-06 11:24:06 +00:00
Mike Barcroft
abbd890233 o Merge <machine/ansi.h> and <machine/types.h> into a new header
called <machine/_types.h>.
o <machine/ansi.h> will continue to live so it can define MD clock
  macros, which are only MD because of gratuitous differences between
  architectures.
o Change all headers to make use of this.  This mainly involves
  changing:
    #ifdef _BSD_FOO_T_
    typedef	_BSD_FOO_T_	foo_t;
    #undef _BSD_FOO_T_
    #endif
  to:
    #ifndef _FOO_T_DECLARED
    typedef	__foo_t	foo_t;
    #define	_FOO_T_DECLARED
    #endif

Concept by:	bde
Reviewed by:	jake, obrien
2002-08-21 16:20:02 +00:00
Robert Drehmel
840b798c83 - Add the 'restrict' qualifier to match the IEEE Std 1003.1-2001
prototype of the tdelete(3) function.
 - Remove duplicated space.
 - Use an ANSI-C function definition for tdelete(3).
 - Update the manual page.
2002-08-14 21:16:41 +00:00
Warner Losh
bb28f3c29b Breath deep and take __P out of the system include files.
# This appears to not break X11, but I'm having problems compiling the
# glide part of the server with or without this patch, so I can't tell
# for sure.
2002-03-23 17:24:55 +00:00
Alfred Perlstein
e31830e941 fix spelling errors.
Pointed out by: bde
2000-07-07 16:52:24 +00:00
Alfred Perlstein
55241422d6 cleanup the tsearch import.
remove (comment out) functions defined or depricated elsewhere:
  bsearch, lfind, lsearch, insque, remque

change hcreate to take a size_t rather than uint (essentially the same)

since hcreate/hdestroy are now in <search.h>, remove private search.h
in lib/libc/db/hash/

add $FreeBSD tags to hsearch.c
2000-07-06 20:04:34 +00:00
Alfred Perlstein
64566a3e2a bring in binary search tree code.
Obtained from: NetBSD
2000-07-01 06:55:11 +00:00