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comments, minor changes

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mysys/lf_alloc-pin.c:
  comments
mysys/lf_dynarray.c:
  comments
mysys/lf_hash.c:
  comments, charset-aware comparison
storage/maria/trnman.c:
  comments
storage/maria/unittest/lockman-t.c:
  test case for a bug
unittest/mysys/my_atomic-t.c:
  removed mistakenly copied line
This commit is contained in:
unknown
2006-10-27 17:09:31 +02:00
parent ce707d9f7f
commit 7ca33ae5b5
6 changed files with 140 additions and 62 deletions
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64
mysys/lf_alloc-pin.c
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@@ -1,3 +1,4 @@
// TODO multi-pinbox
/* Copyright (C) 2000 MySQL AB
This program is free software; you can redistribute it and/or modify
@@ -17,24 +18,25 @@
/*
wait-free concurrent allocator based on pinning addresses
It works as follows: every thread (strictly speaking - every CPU, but it's
too difficult to do) has a small array of pointers. They're called "pins".
Before using an object its address must be stored in this array (pinned).
When an object is no longer necessary its address must be removed from
this array (unpinned). When a thread wants to free() an object it
scans all pins of all threads to see if somebody has this object pinned.
If yes - the object is not freed (but stored in a purgatory).
To reduce the cost of a single free() pins are not scanned on every free()
but only added to (thread-local) purgatory. On every LF_PURGATORY_SIZE
free() purgatory is scanned and all unpinned objects are freed.
It works as follows: every thread (strictly speaking - every CPU, but
it's too difficult to do) has a small array of pointers. They're called
"pins". Before using an object its address must be stored in this array
(pinned). When an object is no longer necessary its address must be
removed from this array (unpinned). When a thread wants to free() an
object it scans all pins of all threads to see if somebody has this
object pinned. If yes - the object is not freed (but stored in a
"purgatory"). To reduce the cost of a single free() pins are not scanned
on every free() but only added to (thread-local) purgatory. On every
LF_PURGATORY_SIZE free() purgatory is scanned and all unpinned objects
are freed.
Pins are used to solve ABA problem. To use pins one must obey
a pinning protocol:
1. Let's assume that PTR is a shared pointer to an object. Shared means
that any thread may modify it anytime to point to a different object and
free the old object. Later the freed object may be potentially allocated
by another thread. If we're unlucky that another thread may set PTR to
point to this object again. This is ABA problem.
that any thread may modify it anytime to point to a different object
and free the old object. Later the freed object may be potentially
allocated by another thread. If we're unlucky that another thread may
set PTR to point to this object again. This is ABA problem.
2. Create a local pointer LOCAL_PTR.
3. Pin the PTR in a loop:
do
@@ -42,31 +44,31 @@
LOCAL_PTR= PTR;
pin(PTR, PIN_NUMBER);
} while (LOCAL_PTR != PTR)
4. It is guaranteed that after the loop is ended, LOCAL_PTR
4. It is guaranteed that after the loop has ended, LOCAL_PTR
points to an object (or NULL, if PTR may be NULL), that
will never be freed. It is not guaranteed though
that LOCAL_PTR == PTR
that LOCAL_PTR == PTR (as PTR can change any time)
5. When done working with the object, remove the pin:
unpin(PIN_NUMBER)
6. When copying pins (as in the list:
6. When copying pins (as in the list traversing loop:
pin(CUR, 1);
while ()
{
pin(CUR, 0);
do
{
NEXT=CUR->next;
pin(NEXT, 1);
} while (NEXT != CUR->next);
do // standard
{ // pinning
NEXT=CUR->next; // loop
pin(NEXT, 0); // see #3
} while (NEXT != CUR->next); // above
...
...
pin(CUR, 1);
CUR=NEXT;
pin(CUR, 1); // copy pin[0] to pin[1]
}
which keeps CUR address constantly pinned), note than pins may be copied
only upwards (!!!), that is pin N to pin M > N.
7. Don't keep the object pinned longer than necessary - the number of pins
you have is limited (and small), keeping an object pinned prevents its
reuse and cause unnecessary mallocs.
which keeps CUR address constantly pinned), note than pins may be
copied only upwards (!!!), that is pin[N] to pin[M], M > N.
7. Don't keep the object pinned longer than necessary - the number of
pins you have is limited (and small), keeping an object pinned
prevents its reuse and cause unnecessary mallocs.
Implementation details:
Pins are given away from a "pinbox". Pinbox is stack-based allocator.
@@ -85,7 +87,7 @@
static void _lf_pinbox_real_free(LF_PINS *pins);
/*
Initialize a pinbox. Must be usually called from lf_alloc_init.
Initialize a pinbox. Normally called from lf_alloc_init.
See the latter for details.
*/
void lf_pinbox_init(LF_PINBOX *pinbox, uint free_ptr_offset,
@@ -214,9 +216,9 @@ static int ptr_cmp(void **a, void **b)
*/
void _lf_pinbox_free(LF_PINS *pins, void *addr)
{
add_to_purgatory(pins, addr);
if (pins->purgatory_count % LF_PURGATORY_SIZE)
_lf_pinbox_real_free(pins);
add_to_purgatory(pins, addr);
}
struct st_harvester {