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

[Nicolai Hähnle]

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;

?

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' ;-)

Cheers,
Nicolai

>        return;
>
>     size = unlink_lru_file_from_directory(dir_path);
>
>     free(dir_path);
>
>     if (size) {
>        p_atomic_add(cache->size, - (uint64_t)size);
>