[Mesa-dev] [PATCH] util/disk_cache: don't require 64bit atomic operations

[Grazvydas Ignotas]

On Sun, Mar 5, 2017 at 10:29 PM, Matt Turner <mattst88 at gmail.com> wrote:
> On Sun, Mar 5, 2017 at 11:14 AM, Grazvydas Ignotas <notasas at gmail.com> wrote:
>> There are still some distributions trying to support unfortunate people
>> with old or exotic CPUs that don't have 64bit atomic operations. Missing
>> 64bit atomic ops can be detected at compile time, so provide a 32bit
>> fallback and add an appropriate cache size limit ceiling for that case.
>>
>> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93089
>> Signed-off-by: Grazvydas Ignotas <notasas at gmail.com>
>
> Again, thanks for working on this.
>
> As I alluded in the other thread, I realize that the typeless
> p_atomics API glosses over a real problem -- that atomic operations
> are no where close to uniformly supported across platforms. libdrm for
> instance, provides an atomic_t type, which is a struct with a single
> int. As far as I can tell, that's really all you can portably do
> atomic operations on.
>
> So I'd like to work towards getting rid of the typeless nature of
> p_atomic*. Until that's fixed, there's not really any way to keep out
> code that's going to break on some platform.
>
> With that in mind...
>
>> ---
>> no commit access
>>
>>  src/util/disk_cache.c | 17 ++++++++++++++++-
>>  1 file changed, 16 insertions(+), 1 deletion(-)
>>
>> diff --git a/src/util/disk_cache.c b/src/util/disk_cache.c
>> index 4a8ee21..daa5c7b 100644
>> --- a/src/util/disk_cache.c
>> +++ b/src/util/disk_cache.c
>> @@ -37,6 +37,7 @@
>>  #include <pwd.h>
>>  #include <errno.h>
>>  #include <dirent.h>
>> +#include <limits.h>
>>  #include "zlib.h"
>>
>>  #include "util/crc32.h"
>> @@ -56,6 +57,11 @@
>>  /* The number of keys that can be stored in the index. */
>>  #define CACHE_INDEX_MAX_KEYS (1 << CACHE_INDEX_KEY_BITS)
>>
>> +/* limit maximum cache size on platforms that lack atomic 64bit operations */
>> +#ifndef __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8
>> +#define CACHE_SIZE_32BIT_LIMITED
>> +#endif
>> +
>>  struct disk_cache {
>>     /* The path to the cache directory. */
>>     char *path;
>> @@ -65,7 +71,11 @@ struct disk_cache {
>>     size_t index_mmap_size;
>>
>>     /* Pointer to total size of all objects in cache (within index_mmap) */
>> +#ifndef CACHE_SIZE_32BIT_LIMITED
>>     uint64_t *size;
>> +#else
>> +   uint32_t *size;
>> +#endif
>
> I talked with Tim on IRC briefly about this. Maybe I'm just showing my
> ignorance, but I'm surprised that we would need more than 4GB address
> space for the shader cache.

>From what I understand, it's not about the address space but about the
total size of all cache files on disk. Deus Ex was reported to use
~50MiB compressed, and that's just to launch the game I guess. With
that in mind, it should be possible to break the 4GiB barrier after
running multiple games if not now then in near future. I think Tim
mentioned Dolphin manages to reach the current 1GiB limit already.

Gražvydas