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Remove the siphash->hmac-md5 switch after 36 bytes.

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Currently, siphash is used for strings up to 36 bytes. hmac-md5 is used
for longer strings.

This switch-over is a remnant of the previous hash-function that was
used, which apparently was slower with longer input strings.

This change serves no purpose anymore. I performed a few performance tests
on strings of varying sizes:

For a 40 byte string with 10 million iterations:

siphash: 0.31 seconds
hmac-md5: 3.8 seconds

For a 1080 byte string with 10 million iterations:

siphash: 4.2 seconds
hmac-md5: 17 seconds

For a 18360 byte string with 10 million iterations:

siphash: 69 seconds
hmac-md5: 240 seconds

Hence, this commit removes the use of hmac-md5.

This change causes reordering of lines in a few logs.

This commit also changes the datastructure for the seed in probabilistic/Hasher
to get rid of a type-punning warning.
This commit is contained in:
Johanna Amann 2020-04-24 13:12:01 -07:00
parent bb050910bb
commit 5e7915ae7a
13 changed files with 269 additions and 297 deletions
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13
src/Hash.cc
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@ -156,15 +156,6 @@ void* HashKey::CopyKey(const void* k, int s) const
hash_t HashKey::HashBytes(const void* bytes, int size)
{
if ( size <= UHASH_KEY_SIZE )
{
hash_t digest = highwayhash::SipHash(shared_siphash_key, reinterpret_cast<const char *>(bytes), size);
return digest;
}
// Fall back to HMAC/MD5 for longer data (which is usually rare).
assert(sizeof(hash_t) == 8);
hash_t digest[2]; // 2x hash_t (uint64_t) = 128 bits = 32 hex chars = sizeof md5
hmac_md5(size, (const unsigned char*) bytes, (unsigned char*) digest);
return digest[0];
hash_t digest = highwayhash::SipHash(shared_siphash_key, reinterpret_cast<const char *>(bytes), size);
return digest;
}