- there's another problem. A bcache stores all its strings in a
- hash table. If you modify a string's contents, you will probably
- change its hash value. This means that the modified string is
- now in the wrong place in the hash table, and future bcache
- probes will never find it. So by mutating a string, you give up
- any chance of sharing its space with future duplicates. */
-
-
-/* The type used to hold a single bcache string. The user data is
- stored in d.data. Since it can be any type, it needs to have the
- same alignment as the most strict alignment of any type on the host
- machine. I don't know of any really correct way to do this in
- stock ANSI C, so just do it the same way obstack.h does.
-
- It would be nicer to have this stuff hidden away in bcache.c, but
- struct objstack contains a struct bcache directly --- not a pointer
- to one --- and then the memory-mapped stuff makes this a real pain.
- We don't strictly need to expose struct bstring, but it's better to
- have it all in one place. */
-
-struct bstring {
- struct bstring *next;
- size_t length;
-
- union
- {
- char data[1];
- double dummy;
- }
- d;
-};
-
-
-/* The structure for a bcache itself.
- To initialize a bcache, just fill it with zeros. */
-struct bcache {
- /* All the bstrings are allocated here. */
- struct obstack cache;
-
- /* How many hash buckets we're using. */
- unsigned int num_buckets;
+ there's another problem. A bcache stores all its strings in a hash
+ table. If you modify a string's contents, you will probably change
+ its hash value. This means that the modified string is now in the
+ wrong place in the hash table, and future bcache probes will never
+ find it. So by mutating a string, you give up any chance of
+ sharing its space with future duplicates.
+
+
+ Size of bcache VS hashtab:
+
+ For bcache, the most critical cost is size (or more exactly the
+ overhead added by the bcache). It turns out that the bcache is
+ remarkably efficient.
+
+ Assuming a 32-bit system (the hash table slots are 4 bytes),
+ ignoring alignment, and limit strings to 255 bytes (1 byte length)
+ we get ...
+
+ bcache: This uses a separate linked list to track the hash chain.
+ The numbers show roughly 100% occupancy of the hash table and an
+ average chain length of 4. Spreading the slot cost over the 4
+ chain elements:
+
+ 4 (slot) / 4 (chain length) + 1 (length) + 4 (chain) = 6 bytes
+
+ hashtab: This uses a more traditional re-hash algorithm where the
+ chain is maintained within the hash table. The table occupancy is
+ kept below 75% but we'll assume its perfect:
+
+ 4 (slot) x 4/3 (occupancy) + 1 (length) = 6 1/3 bytes
+
+ So a perfect hashtab has just slightly larger than an average
+ bcache.
+
+ It turns out that an average hashtab is far worse. Two things
+ hurt:
+
+ - Hashtab's occupancy is more like 50% (it ranges between 38% and
+ 75%) giving a per slot cost of 4x2 vs 4x4/3.
+
+ - the string structure needs to be aligned to 8 bytes which for
+ hashtab wastes 7 bytes, while for bcache wastes only 3.
+
+ This gives:
+
+ hashtab: 4 x 2 + 1 + 7 = 16 bytes
+
+ bcache 4 / 4 + 1 + 4 + 3 = 9 bytes
+
+ The numbers of GDB debugging GDB support this. ~40% vs ~70% overhead.
+
+
+ Speed of bcache VS hashtab (the half hash hack):
+
+ While hashtab has a typical chain length of 1, bcache has a chain
+ length of round 4. This means that the bcache will require
+ something like double the number of compares after that initial
+ hash. In both cases the comparison takes the form:
+
+ a.length == b.length && memcmp (a.data, b.data, a.length) == 0
+
+ That is lengths are checked before doing the memcmp.
+
+ For GDB debugging GDB, it turned out that all lengths were 24 bytes
+ (no C++ so only psymbols were cached) and hence, all compares
+ required a call to memcmp. As a hack, two bytes of padding
+ (mentioned above) are used to store the upper 16 bits of the
+ string's hash value and then that is used in the comparison vis:
+
+ a.half_hash == b.half_hash && a.length == b.length && memcmp
+ (a.data, b.data, a.length)
+
+ The numbers from GDB debugging GDB show this to be a remarkable
+ 100% effective (only necessary length and memcmp tests being
+ performed).
+
+ Mind you, looking at the wall clock, the same GDB debugging GDB
+ showed only marginal speed up (0.780 vs 0.773s). Seems GDB is too
+ busy doing something else :-(
projects / deliverable / binutils-gdb.git / blobdiff