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Replaced separate BLOCKSIZE/BLOCKCOUNT attrs with single GEOMETRY attr

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This saves a bit of rbyd overhead, since these almost always come
together.

Perhaps more interesting, it carves out space for storing mroot-anchor
redundancy information. This uses the lowest two bits of the GEOMETRY
tag to indicate how many redundant blocks belong to the mroot-anchor:

  LFSR_TAG_GEOMETRY       0x0008  v--- ---- ---- 1-rr

This solves a bit of a hole in our redundancy encoding. The plan is for
this info to be stored in the lowest two bits of every pointer, but the
mroot-anchor doesn't really have a pointer.

Though this is just future plans. Right now the redundancy information
is unused. Current implementations should use the GEOMETRY tag 0x0009,
which you may notice implied redundancy level-1. This matches our
current 2-block per mdir default.

Geometry attr encoding:

  .---+---+---+---.      tag (0x0008+r): 1 be16    2 bytes
  |x0008+r| 0 |siz|      weight (0):     1 leb128  1 byte
  +---+---+---+---+      size:           1 leb128  1 byte
  | block_size    |      block_size:     1 leb128  <=4 bytes
  +---+- -+- -+- -+- -.
  | block_count       |  block_count:    1 leb128  <=5 bytes
  '---+- -+- -+- -+- -'  total:                    <=13 bytes

Code changes:

           code          stack
  before: 34092           2880
  after:  34040 (-0.2%)   2880 (+0.0%)
This commit is contained in:
Christopher Haster
2024-03-18 02:15:46 -05:00
parent 796be705ac
commit 9366674416
6 changed files with 105 additions and 91 deletions
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scripts/dbgbmap.py
+3 -4
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@@ -17,10 +17,9 @@ TAG_VERSION = 0x0004
TAG_RCOMPAT = 0x0005
TAG_WCOMPAT = 0x0006
TAG_OCOMPAT = 0x0007
TAG_BLOCKSIZE = 0x0008
TAG_BLOCKCOUNT = 0x0009
TAG_NAMELIMIT = 0x000a
TAG_SIZELIMIT = 0x000b
TAG_GEOMETRY = 0x0009
TAG_NAMELIMIT = 0x000c
TAG_SIZELIMIT = 0x000d
TAG_GDELTA = 0x0100
TAG_GRMDELTA = 0x0100
TAG_NAME = 0x0200