diff options
| author | Sergey Bugaev <bugaevc@gmail.com> | 2023-06-26 14:26:50 +0300 |
|---|---|---|
| committer | Samuel Thibault <samuel.thibault@ens-lyon.org> | 2023-07-03 01:59:49 +0200 |
| commit | 505ecd7b30fca1dc296205ff3c0dc3aff9d76e12 (patch) | |
| tree | f19cb99981292863d2db5ae6a0ad43e559b3acac /vm/vm_page.h | |
| parent | d5464d531aaba049a3e734adb80fb0bfd28093a7 (diff) | |
| download | gnumach-505ecd7b30fca1dc296205ff3c0dc3aff9d76e12.tar.gz gnumach-505ecd7b30fca1dc296205ff3c0dc3aff9d76e12.tar.bz2 gnumach-505ecd7b30fca1dc296205ff3c0dc3aff9d76e12.zip | |
Shrink struct vm_page size
struct vm_page is supposed to be a "small structure", but it takes up 96
bytes on x86_64 (to represent a 4k page). By utilizing bitfields and
strategically reordering members to avoid excessive padding, it can be
shrunk to 80 bytes.
- page_lock and unlock_request only need to store a bitmask of
VM_PROT_READ, VM_PROT_WRITE, and VM_PROT_EXECUTE. Even though the
special values VM_PROT_NO_CHANGE and VM_PROT_NOTIFY are defined, they
are not used for the two struct vm_page members.
- type and seg_index both need to store one of the four possible values
in the range from 0 to 3. Two bits are sufficient for this.
- order needs to store a number from 0 to VM_PAGE_NR_FREE_LISTS (which
is 11), or a special value VM_PAGE_ORDER_UNLISTED. Four bits are
sufficient for this.
No functional change.
Message-Id: <20230626112656.435622-2-bugaevc@gmail.com>
Diffstat (limited to 'vm/vm_page.h')
| -rw-r--r-- | vm/vm_page.h | 18 |
1 files changed, 12 insertions, 6 deletions
diff --git a/vm/vm_page.h b/vm/vm_page.h index d457f9a2..b2581d9e 100644 --- a/vm/vm_page.h +++ b/vm/vm_page.h @@ -79,9 +79,6 @@ struct vm_page { struct list node; /* page queues or free list (P) */ - unsigned short type; - unsigned short seg_index; - unsigned short order; void *priv; /* @@ -95,7 +92,6 @@ struct vm_page { /* We use an empty struct as the delimiter. */ struct {} vm_page_header; -#define VM_PAGE_HEADER_SIZE offsetof(struct vm_page, vm_page_header) vm_object_t object; /* which object am I in (O,P) */ vm_offset_t offset; /* offset into that object (O,P) */ @@ -126,10 +122,20 @@ struct vm_page { * without having data. (O) * [See vm_object_overwrite] */ - vm_prot_t page_lock; /* Uses prohibited by data manager (O) */ - vm_prot_t unlock_request; /* Outstanding unlock request (O) */ + vm_prot_t page_lock:3; /* Uses prohibited by data manager (O) */ + vm_prot_t unlock_request:3; /* Outstanding unlock request (O) */ + + struct {} vm_page_footer; + + unsigned short type:2; + unsigned short seg_index:2; + unsigned short order:4; }; +#define VM_PAGE_BODY_SIZE \ + (offsetof(struct vm_page, vm_page_footer) \ + - offsetof(struct vm_page, vm_page_header)) + /* * For debugging, this macro can be defined to perform * some useful check on a page structure. |