[Mesa-dev] [PATCH 3/3] util/disk_cache: use rand_xorshift128plus() to get our random int

[Timothy Arceri]

On 21/03/17 21:50, Timothy Arceri wrote:
>
>
> On 21/03/17 21:10, Nicolai Hähnle wrote:
>> On 21.03.2017 09:57, Timothy Arceri wrote:
>>> Otherwise for apps that don't seed the regular rand() we will always
>>> remove old cache entries from the same dirs.
>>> ---
>>>  src/util/disk_cache.c | 9 +++++++--
>>>  1 file changed, 7 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/src/util/disk_cache.c b/src/util/disk_cache.c
>>> index f2d67c9..f6ecf0b 100644
>>> --- a/src/util/disk_cache.c
>>> +++ b/src/util/disk_cache.c
>>> @@ -34,20 +34,21 @@
>>>  #include <sys/statvfs.h>
>>>  #include <sys/mman.h>
>>>  #include <unistd.h>
>>>  #include <fcntl.h>
>>>  #include <pwd.h>
>>>  #include <errno.h>
>>>  #include <dirent.h>
>>>  #include "zlib.h"
>>>
>>>  #include "util/crc32.h"
>>> +#include "util/rand_xor.h"
>>>  #include "util/u_atomic.h"
>>>  #include "util/u_queue.h"
>>>  #include "util/mesa-sha1.h"
>>>  #include "util/ralloc.h"
>>>  #include "main/errors.h"
>>>  #include "util/macros.h"
>>>
>>>  #include "disk_cache.h"
>>>
>>>  /* Number of bits to mask off from a cache key to get an index. */
>>> @@ -354,20 +355,23 @@ disk_cache_create(const char *gpu_name, const
>>> char *timestamp)
>>>     cache->max_size = max_size;
>>>
>>>     /* A limit of 32 jobs was choosen as observations of Deus Ex
>>> start-up times
>>>      * showed that we reached at most 11 jobs on an Intel i5-6400
>>> CPU at 2.70GHz
>>>      * (a fairly modest desktop CPU). 1 thread was chosen because we
>>> don't
>>>      * really care about getting things to disk quickly just that it's
>>> not
>>>      * blocking other tasks.
>>>      */
>>>     util_queue_init(&cache->cache_queue, "disk_cache", 32, 1);
>>>
>>> +   /* Seed our rand function */
>>> +   s_rand_xorshift128plus();
>>> +
>>>     ralloc_free(local);
>>>
>>>     return cache;
>>>
>>>   fail:
>>>     if (fd != -1)
>>>        close(fd);
>>>     if (cache)
>>>        ralloc_free(cache);
>>>     ralloc_free(local);
>>> @@ -569,22 +573,23 @@ evict_lru_item(struct disk_cache *cache)
>>>     const char hex[] = "0123456789abcde";
>>>     char *dir_path;
>>>     int a, b;
>>>     size_t size;
>>>
>>>     /* With a reasonably-sized, full cache, (and with keys generated
>>>      * from a cryptographic hash), we can choose two random hex digits
>>>      * and reasonably expect the directory to exist with a file in it.
>>>      * Provides pseudo-LRU eviction to reduce checking all cache files.
>>>      */
>>> -   a = rand() % 16;
>>> -   b = rand() % 16;
>>> +   uint64_t rand64 = rand_xorshift128plus();
>>> +   a = (rand64 >> 32) % 16;
>>> +   b = (rand64 & 0xFFFFFFFF) % 16;
>>
>> Why not just
>>
>> a = rand64 & 0xf;
>> b = (rand64 >> 4) & 0xf;
>>
>> ?
>
> Because % 16 is meant to give you a uniform distribution of random
> numbers between 0-15. Thinking about this some more I don't think I
> should really be splitting the number in half. I should really call it
> twice, I was looking for an open-source 32-bit generator but only found
> papers not code so settled with this existing one.

Actually it should probably just be % 32, getting the two letters from a 
single call. Or possibly % 33 with the modulo bias remove or some other 
workaround to keep uniform distribution so that we fix the missing upper 
limited you pointed out below.

>
>>
>> Actually, why not simplify even further by saying:
>>
>>>
>>>     if (asprintf(&dir_path, "%s/%c%c", cache->path, hex[a], hex[b]) < 0)
>>
>> asprintf(..., "%s/%02x", ..., rand64 & 0xff)
>>
>> That would also fix the bug that the hex string is missing the final 'f'
>> ;-)
>
> Huh, hadn't noticed that. The joys of finish off prototype code :)
>
>>
>> Cheers,
>> Nicolai
>>
>>>        return;
>>>
>>>     size = unlink_lru_file_from_directory(dir_path);
>>>
>>>     free(dir_path);
>>>
>>>     if (size) {
>>>        p_atomic_add(cache->size, - (uint64_t)size);
>>>
>>
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