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SC: Simplify SC_NPSIZES computation.

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Rather than taking all the sizes and subtracting out those that don't fit, we
instead just add up all the ones that do.
This commit is contained in:
David Goldblatt 2020-04-14 18:32:54 -07:00 committed by David Goldblatt
parent fb6cfffd39
commit 79dd0c04ed
1 changed files with 26 additions and 22 deletions
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48
include/jemalloc/internal/sc.h
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@ -197,30 +197,34 @@
(SC_LG_BASE_MAX - SC_LG_FIRST_REGULAR_BASE + 1) - 1) (SC_LG_BASE_MAX - SC_LG_FIRST_REGULAR_BASE + 1) - 1)
#define SC_NSIZES (SC_NTINY + SC_NPSEUDO + SC_NREGULAR) #define SC_NSIZES (SC_NTINY + SC_NPSEUDO + SC_NREGULAR)
/* The number of size classes that are a multiple of the page size. */
#define SC_NPSIZES ( \
/* Start with all the size classes. */ \
SC_NSIZES \
/* Subtract out those groups with too small a base. */ \
- (LG_PAGE - 1 - SC_LG_FIRST_REGULAR_BASE) * SC_NGROUP \
/* And the pseudo-group. */ \
- SC_NPSEUDO \
/* And the tiny group. */ \
- SC_NTINY \
/* Sizes where ndelta*delta is not a multiple of the page size. */ \
- (SC_LG_NGROUP * SC_NGROUP))
/* /*
* Note that the last line is computed as the sum of the second column in the * The number of size classes that are a multiple of the page size.
* following table: *
* lg(base) | count of sizes to exclude * Here are the first few bases that have a page-sized SC.
* ------------------------------|----------------------------- *
* LG_PAGE - 1 | SC_NGROUP - 1 * lg(base) | base | highest SC | page-multiple SCs
* LG_PAGE | SC_NGROUP - 1 * --------------|------------------------------------------
* LG_PAGE + 1 | SC_NGROUP - 2 * LG_PAGE - 1 | PAGE / 2 | PAGE | 1
* LG_PAGE + 2 | SC_NGROUP - 4 * LG_PAGE | PAGE | 2 * PAGE | 1
* ... | ... * LG_PAGE + 1 | 2 * PAGE | 4 * PAGE | 2
* LG_PAGE + (SC_LG_NGROUP - 1) | SC_NGROUP - (SC_NGROUP / 2) * LG_PAGE + 2 | 4 * PAGE | 8 * PAGE | 4
*
* The number of page-multiple SCs continues to grow in powers of two, up until
* lg_delta == lg_page, which corresponds to setting lg_base to lg_page +
* SC_LG_NGROUP. So, then, the number of size classes that are multiples of the
* page size whose lg_delta is less than the page size are
* is 1 + (2**0 + 2**1 + ... + 2**(lg_ngroup - 1) == 2**lg_ngroup.
*
* For each base with lg_base in [lg_page + lg_ngroup, lg_base_max), there are
* NGROUP page-sized size classes, and when lg_base == lg_base_max, there are
* NGROUP - 1.
*
* This gives us the quantity we seek.
*/ */
#define SC_NPSIZES ( \
SC_NGROUP \
+ (SC_LG_BASE_MAX - (LG_PAGE + SC_LG_NGROUP)) * SC_NGROUP \
+ SC_NGROUP - 1)
/* /*
* We declare a size class is binnable if size < page size * group. Or, in other * We declare a size class is binnable if size < page size * group. Or, in other