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Separate completely full runs from runs that are merely almost full, so

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that no linear searching is necessary if we resort to allocating from a
run that is known to be mostly full.  There are pathological edge cases
that could have caused severely degraded performance, and this change
fixes that.
This commit is contained in:
jasone 2006-03-20 04:05:05 +00:00
parent 020594940d
commit 8a77abffbc
1 changed files with 71 additions and 61 deletions
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132
lib/libc/stdlib/malloc.c
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@ -465,14 +465,15 @@ struct arena_run_s {
unsigned nfree:(RUN_MIN_REGS_2POW + 1);
/*
* Current quartile for this run, one of: {RUN_Q0, RUN_25, RUN_Q50,
* RUN_Q75, RUN_Q100}.
* Current quartile for this run, one of: {RUN_QEMPTY, RUN_Q0, RUN_25,
* RUN_Q50, RUN_Q75, RUN_Q100}.
*/
#define RUN_Q0 0
#define RUN_Q25 1
#define RUN_Q50 2
#define RUN_Q75 3
#define RUN_Q100 4
#define RUN_QEMPTY 0
#define RUN_Q0 1
#define RUN_Q25 2
#define RUN_Q50 3
#define RUN_Q75 4
#define RUN_Q100 5
unsigned quartile:3;
/*
@ -492,35 +493,38 @@ struct arena_bin_s {
arena_run_t *runcur;
/*
* Links into rings of runs, of various fullnesses. A new run
* conceptually starts off in runs0, and when it reaches 25% full, it
* is moved to the runs25 ring. For the run to be moved again, it must
* become either empty or 50% full. Thus, each ring contains runs that
* are within 25% of the advertised fullness for the ring. This
* provides a low-overhead mechanism for segregating runs into
* approximate fullness classes.
* Links into rings of runs, of various fullnesses (names indicate
* approximate lower bounds). A new run conceptually starts off in
* runsempty, and it isn't inserted into the runs0 ring until it
* reaches 25% full (hysteresis mechanism). For the run to be moved
* again, it must become either empty or 50% full. Thus, each ring
* contains runs that are within 50% above the advertised fullness for
* the ring. This provides a low-overhead mechanism for segregating
* runs into approximate fullness classes.
*
* These rings are useful when looking for an existing run
* to use when runcur is no longer usable. We look for usable runs in
* the following order:
* Conceptually, there is a runs100 that contains completely full runs.
* Since we don't need to search for these runs though, no runs100 ring
* is actually maintained.
*
* 1) runs75
* 2) runs50
* 3) runs25
* 4) runs100
* These rings are useful when looking for an existing run to use when
* runcur is no longer usable. We look for usable runs in the
* following order:
*
* We never look in runs0 because it never has more than one run in it,
* and in such cases runcur already points to that run.
* 1) runs50
* 2) runs25
* 3) runs0
* 4) runs75
*
* runs100 isn't a good place to look, because it contains runs that
* may be completely full. Still, we look there as a last resort in
* order to avoid allocating a new run if at all possible.
* runs75 isn't a good place to look, because it contains runs that
* may be nearly completely full. Still, we look there as a last
* resort in order to avoid allocating a new run if at all possible.
*/
/* arena_run_t runs0; 0% <= fullness < 25% */
arena_run_t runs25; /* 0% < fullness < 50% */
arena_run_t runs50; /* 25% < fullness < 75% */
arena_run_t runs75; /* 50% < fullness < 100% */
arena_run_t runs100; /* 75% < fullness <= 100% */
/* arena_run_t runsempty; 0% <= fullness < 25% */
arena_run_t runs0; /* 0% < fullness < 50% */
arena_run_t runs25; /* 25% < fullness < 75% */
arena_run_t runs50; /* 50% < fullness < 100% */
arena_run_t runs75; /* 75% < fullness < 100% */
/* arena_run_t runs100; fullness == 100% */
/* Size of regions in a run for this bin's size class. */
size_t reg_size;
@ -1551,13 +1555,25 @@ arena_bin_run_refile(arena_t *arena, arena_bin_t *bin, arena_run_t *run,
assert(run->free_min > run->nfree);
assert(run->quartile < RUN_Q100);
run->quartile++;
if (run->quartile == RUN_Q75) {
/*
* Skip RUN_Q75 during promotion from RUN_Q50.
* Separate handling of RUN_Q75 and RUN_Q100 allows
* us to keep completely full runs in RUN_Q100, thus
* guaranteeing that runs in RUN_Q75 are only mostly
* full. This provides a method for avoiding a linear
* search for non-full runs, which avoids some
* pathological edge cases.
*/
run->quartile++;
}
#ifdef MALLOC_STATS
bin->stats.npromote++;
#endif
} else {
/* Demote. */
assert(run->free_max < run->nfree);
assert(run->quartile > RUN_Q0);
assert(run->quartile > RUN_QEMPTY);
run->quartile--;
#ifdef MALLOC_STATS
bin->stats.ndemote++;
@ -1567,7 +1583,7 @@ arena_bin_run_refile(arena_t *arena, arena_bin_t *bin, arena_run_t *run,
/* Re-file run. */
qr_remove(run, link);
switch (run->quartile) {
case RUN_Q0:
case RUN_QEMPTY:
#ifdef MALLOC_STATS
bin->stats.curruns--;
#endif
@ -1578,26 +1594,30 @@ arena_bin_run_refile(arena_t *arena, arena_bin_t *bin, arena_run_t *run,
#endif
arena_run_dalloc(arena, run, bin->run_size);
break;
case RUN_Q25:
qr_before_insert(&bin->runs25, run, link);
case RUN_Q0:
qr_before_insert(&bin->runs0, run, link);
run->free_max = run->bin->nregs - 1;
run->free_min = (run->bin->nregs >> 1) + 1;
break;
case RUN_Q50:
qr_before_insert(&bin->runs50, run, link);
case RUN_Q25:
qr_before_insert(&bin->runs25, run, link);
run->free_max = ((run->bin->nregs >> 2) * 3) - 1;
run->free_min = (run->bin->nregs >> 2) + 1;
break;
case RUN_Q50:
qr_before_insert(&bin->runs50, run, link);
run->free_max = (run->bin->nregs >> 1) - 1;
run->free_min = 1;
break;
case RUN_Q75:
qr_before_insert(&bin->runs75, run, link);
run->free_max = (run->bin->nregs >> 1) - 1;
run->free_max = (run->bin->nregs >> 2) - 1;
run->free_min = 1;
break;
case RUN_Q100:
assert(bin->runcur == run);
bin->runcur = NULL;
qr_before_insert(&bin->runs100, run, link);
run->free_max = (run->bin->nregs >> 2) - 1;
run->free_max = 0;
run->free_min = 0;
break;
default:
@ -1752,26 +1772,15 @@ arena_bin_nonfull_run_get(arena_t *arena, arena_bin_t *bin, size_t size)
unsigned i, remainder;
/* Look for a usable run. */
if ((run = qr_next(&bin->runs75, link)) != &bin->runs75
|| (run = qr_next(&bin->runs50, link)) != &bin->runs50
|| (run = qr_next(&bin->runs25, link)) != &bin->runs25) {
/* Use a run that is guaranteed to have available space. */
if ((run = qr_next(&bin->runs50, link)) != &bin->runs50
|| (run = qr_next(&bin->runs25, link)) != &bin->runs25
|| (run = qr_next(&bin->runs0, link)) != &bin->runs0
|| (run = qr_next(&bin->runs75, link)) != &bin->runs75) {
/* run is guaranteed to have available space. */
qr_remove(run, link);
return (run);
}
/* Look for a run in runs100 that has available space. */
if ((run = qr_next(&bin->runs100, link)) != &bin->runs100) {
do {
if (run->nfree > 0) {
qr_remove(run, link);
return (run);
}
run = qr_next(run, link);
} while (run != &bin->runs100);
}
/* Allocate a new run. */
run = arena_run_alloc(arena, false, bin->run_size);
if (run == NULL)
@ -1795,7 +1804,7 @@ arena_bin_nonfull_run_get(arena_t *arena, arena_bin_t *bin, size_t size)
run->regs_minelm = 0;
run->nfree = bin->nregs;
run->quartile = RUN_Q0;
run->quartile = RUN_QEMPTY;
run->free_max = bin->nregs;
run->free_min = ((bin->nregs >> 2) * 3) + 1;
#ifdef MALLOC_DEBUG
@ -1819,6 +1828,7 @@ arena_bin_malloc_easy(arena_t *arena, arena_bin_t *bin, arena_run_t *run,
unsigned regind;
assert(run->magic == ARENA_RUN_MAGIC);
assert(run->nfree > 0);
regind = arena_run_search(run);
assert(regind != UINT_MAX);
@ -1887,7 +1897,7 @@ arena_malloc(arena_t *arena, size_t size)
}
assert(size == bin->reg_size);
if ((run = bin->runcur) != NULL && run->nfree > 0)
if ((run = bin->runcur) != NULL)
ret = arena_bin_malloc_easy(arena, bin, run, size);
else
ret = arena_bin_malloc_hard(arena, bin, size);
@ -2098,10 +2108,10 @@ arena_new(arena_t *arena)
for (i = 0; i < ntbins; i++) {
bin = &arena->bins[i];
bin->runcur = NULL;
qr_new(&bin->runs0, link);
qr_new(&bin->runs25, link);
qr_new(&bin->runs50, link);
qr_new(&bin->runs75, link);
qr_new(&bin->runs100, link);
bin->reg_size = (1 << (TINY_MIN_2POW + i));
@ -2135,10 +2145,10 @@ arena_new(arena_t *arena)
for (; i < ntbins + nqbins; i++) {
bin = &arena->bins[i];
bin->runcur = NULL;
qr_new(&bin->runs0, link);
qr_new(&bin->runs25, link);
qr_new(&bin->runs50, link);
qr_new(&bin->runs75, link);
qr_new(&bin->runs100, link);
bin->reg_size = quantum * (i - ntbins + 1);
@ -2169,10 +2179,10 @@ arena_new(arena_t *arena)
for (; i < ntbins + nqbins + npbins; i++) {
bin = &arena->bins[i];
bin->runcur = NULL;
qr_new(&bin->runs0, link);
qr_new(&bin->runs25, link);
qr_new(&bin->runs50, link);
qr_new(&bin->runs75, link);
qr_new(&bin->runs100, link);
bin->reg_size = (small_max << (i - (ntbins + nqbins) + 1));