increase hashtable size for packages/groups by factor 5

It also makes the size configureable, so it can be adapted in the future
without the need for an abi break - and even by users…

The increase was long overdue as it gives a >10% decrease in runtime of
e.g. 'apt-get check -s'. Some (useless) benchmark with 69933 groups and
187796 packages without a pre-built cache:
time apt-get check -so APT::Cache-HashTableSize=1 → 20m
time apt-get check -so APT::Cache-HashTableSize=1000 → 6,41s
time apt-get check -so APT::Cache-HashTableSize=2000 → 5,64s (old)
time apt-get check -so APT::Cache-HashTableSize=3000 → 5,30s
time apt-get check -so APT::Cache-HashTableSize=5000 → 5,08s
time apt-get check -so APT::Cache-HashTableSize=6000 → 5,05s
time apt-get check -so APT::Cache-HashTableSize=7000 → 5,02s
time apt-get check -so APT::Cache-HashTableSize=8000 → 5,00s
time apt-get check -so APT::Cache-HashTableSize=9000 → 4,98s
time apt-get check -so APT::Cache-HashTableSize=10000 → 4,96s (new)
time apt-get check -so APT::Cache-HashTableSize=15000 → 4,90s
time apt-get check -so APT::Cache-HashTableSize=20000 → 4,86s
time apt-get check -so APT::Cache-HashTableSize=30000 → 4,77s
time apt-get check -so APT::Cache-HashTableSize=40000 → 4,74s
time apt-get check -so APT::Cache-HashTableSize=50000 → 4,73s
time apt-get check -so APT::Cache-HashTableSize=60000 → 4,71s

The gap increases further for operations which have more package
lookups. Factor 5 was chosen as higher values do not provide any
really significant timing advantage anymore compared to the memory
increase in my testing and there is always the possibility to increase
it now if that changes. (also most users will not have 3 releases and
4 architectures in the cache, so theirs will be much smaller and faster).

1 files changed, 6 insertions, 6 deletions

diff --git a/apt-pkg/pkgcache.h b/apt-pkg/pkgcache.h
index 55f0187f9..b6b2894cf 100644
--- a/apt-pkg/pkgcache.h
+++ b/apt-pkg/pkgcache.h
@@ -304,20 +304,20 @@ struct pkgCache::Header
        stores this information so future additions can make use of any unused pool
        blocks. */
    DynamicMMap::Pool Pools[9];
-   
+
    /** \brief hash tables providing rapid group/package name lookup
 
-       Each group/package name is inserted into the hash table using pkgCache::Hash(const &string)
+       Each group/package name is inserted into a hash table using pkgCache::Hash(const &string)
        By iterating over each entry in the hash table it is possible to iterate over
        the entire list of packages. Hash Collisions are handled with a singly linked
        list of packages based at the hash item. The linked list contains only
        packages that match the hashing function.
        In the PkgHashTable is it possible that multiple packages have the same name -
        these packages are stored as a sequence in the list.
-
-       Beware: The Hashmethod assumes that the hash table sizes are equal */
-   map_ptrloc PkgHashTable[64*1048];
-   map_ptrloc GrpHashTable[64*1048];
+       The size of both tables is the same. */
+   unsigned int HashTableSize;
+   map_ptrloc * PkgHashTable() const { return (map_ptrloc*) (this + 1); }
+   map_ptrloc * GrpHashTable() const { return PkgHashTable() + HashTableSize; }
 
    /** \brief Size of the complete cache file */
    unsigned long  CacheFileSize;