On (02/19/16 13:46), Sergey Senozhatsky wrote:
> On (02/19/16 13:16), Sergey Senozhatsky wrote:
> > ok, this sets us on a "do we need 32 and 48 bytes classes at all" track?
> >
>
> seems that lz4 defines a minimum length to be at least
>
> 61 #define COPYLENGTH 8
> 67 #define MINMATCH 4
> 70 #define MFLIMIT (COPYLENGTH + MINMATCH)
> 71 #define MINLENGTH (MFLIMIT + 1)
>
> bytes.
hm, on a second look, zsmalloc defines the following macros:
#define ZS_MAX_ZSPAGE_ORDER 2
#define ZS_MAX_PAGES_PER_ZSPAGE (_AC(1, UL) << ZS_MAX_ZSPAGE_ORDER)
#ifndef MAX_PHYSMEM_BITS
#ifdef CONFIG_HIGHMEM64G
#define MAX_PHYSMEM_BITS 36
#else /* !CONFIG_HIGHMEM64G */
/*
* If this definition of MAX_PHYSMEM_BITS is used, OBJ_INDEX_BITS will just
* be PAGE_SHIFT
*/
#define MAX_PHYSMEM_BITS BITS_PER_LONG
#endif
#endif
#define _PFN_BITS (MAX_PHYSMEM_BITS - PAGE_SHIFT)
#define OBJ_ALLOCATED_TAG 1
#define OBJ_TAG_BITS 1
#define OBJ_INDEX_BITS (BITS_PER_LONG - _PFN_BITS - OBJ_TAG_BITS)
#define OBJ_INDEX_MASK ((_AC(1, UL) << OBJ_INDEX_BITS) - 1)
#define ZS_MIN_ALLOC_SIZE \
MAX(32, (ZS_MAX_PAGES_PER_ZSPAGE << PAGE_SHIFT >> OBJ_INDEX_BITS))
so let's do some calculations, hopefuly I'm not mistaken anywhere.
with ZS_MAX_ZSPAGE_ORDER 4
-- on 32 bit system, PAGE_SHIFT 12
ZS_MAX_PAGES_PER_ZSPAGE 1 << 4 16
OBJ_INDEX_BITS (32 - (32 - 12) - 1) 11
OBJ_INDEX_MASK ((1 << (32 - (32 - 12) - 1)) - 1) 2047
ZS_MIN_ALLOC_SIZE MAX(32, ((1 << 4) << 12 >> (32 - (32 - 12) - 1))) 32
-- on 64 bit system, PAGE_SHIFT 12
ZS_MAX_PAGES_PER_ZSPAGE 1 << 4 16
OBJ_INDEX_BITS (64 - (64 - 12) - 1) 11
OBJ_INDEX_MASK ((1 << (64 - (64 - 12) - 1)) - 1) 2047
ZS_MIN_ALLOC_SIZE MAX(32, ((1 << 4) << 12 >> (64 - (64 - 12) - 1))) 32
-- on 64 bit system, PAGE_SHIFT 14
ZS_MAX_PAGES_PER_ZSPAGE 1 << 4 16
OBJ_INDEX_BITS (64 - (64 - 14) - 1) 13
OBJ_INDEX_MASK ((1 << (64 - (64 - 14) - 1)) - 1) 8191
ZS_MIN_ALLOC_SIZE MAX(32, ((1 << 4) << 14 >> (64 - (64 - 14) - 1))) 32
-- on 64 bit system, PAGE_SHIFT 16
ZS_MAX_PAGES_PER_ZSPAGE 1 << 4 16
OBJ_INDEX_BITS (64 - (64 - 16) - 1) 15
OBJ_INDEX_MASK ((1 << (64 - (64 - 16) - 1)) - 1) 32767
ZS_MIN_ALLOC_SIZE MAX(32, ((1 << 4) << 16 >> (64 - (64 - 14) - 1))) 128
<< bad
so, isn't it enough OBJ_INDEX_BITS bits to even keep 32 bytes class around?
we probably would prefer to lower ZS_MAX_ZSPAGE_ORDER on PAGE_SHIFT 16 systems.
for example to ZS_MAX_PAGES_PER_ZSPAGE 1 << 3, or 1 << 2.
and of course LPAE/PAE enabled systems -- leave ZS_MAX_ZSPAGE_ORDER 2 there.
ZS_MAX_PAGES_PER_ZSPAGE 1 << 4 gives us
# cat /sys/kernel/debug/zsmalloc/zram0/classes
class size huge almost_full almost_empty obj_allocated obj_used
pages_used pages_per_zspage
0 32 0 0 0 0 0
1
...
238 3840 0 0 0 0 0
15
254 4096 Y 0 0 0 0 0
1
so starting from 3840+ we have huge classes, the rest are 'normal' classes and
will
save memory there in theory.
-ss
