/* allocate some space from the free list. The offset returned points
to a unconnected tdb1_record within the database with room for at
least length bytes of total data
0 is returned if the space could not be allocated
*/
tdb1_off_t tdb1_allocate(struct tdb_context *tdb, tdb1_len_t length, struct tdb1_record *rec)
{
tdb1_off_t rec_ptr, last_ptr, newrec_ptr;
struct {
tdb1_off_t rec_ptr, last_ptr;
tdb1_len_t rec_len;
} bestfit;
float multiplier = 1.0;
if (tdb1_lock(tdb, -1, F_WRLCK) == -1)
return 0;
/* over-allocate to reduce fragmentation */
length *= 1.25;
/* Extra bytes required for tailer */
length += sizeof(tdb1_off_t);
length = TDB1_ALIGN(length, TDB1_ALIGNMENT);
again:
last_ptr = TDB1_FREELIST_TOP;
/* read in the freelist top */
if (tdb1_ofs_read(tdb, TDB1_FREELIST_TOP, &rec_ptr) == -1)
goto fail;
bestfit.rec_ptr = 0;
bestfit.last_ptr = 0;
bestfit.rec_len = 0;
/*
this is a best fit allocation strategy. Originally we used
a first fit strategy, but it suffered from massive fragmentation
issues when faced with a slowly increasing record size.
*/
while (rec_ptr) {
if (tdb1_rec_free_read(tdb, rec_ptr, rec) == -1) {
goto fail;
}
if (rec->rec_len >= length) {
if (bestfit.rec_ptr == 0 ||
rec->rec_len < bestfit.rec_len) {
bestfit.rec_len = rec->rec_len;
bestfit.rec_ptr = rec_ptr;
bestfit.last_ptr = last_ptr;
}
}
/* move to the next record */
last_ptr = rec_ptr;
rec_ptr = rec->next;
/* if we've found a record that is big enough, then
stop searching if its also not too big. The
definition of 'too big' changes as we scan
through */
if (bestfit.rec_len > 0 &&
bestfit.rec_len < length * multiplier) {
break;
}
/* this multiplier means we only extremely rarely
search more than 50 or so records. At 50 records we
accept records up to 11 times larger than what we
want */
multiplier *= 1.05;
}
if (bestfit.rec_ptr != 0) {
if (tdb1_rec_free_read(tdb, bestfit.rec_ptr, rec) == -1) {
goto fail;
}
newrec_ptr = tdb1_allocate_ofs(tdb, length, bestfit.rec_ptr,
rec, bestfit.last_ptr);
tdb1_unlock(tdb, -1, F_WRLCK);
return newrec_ptr;
}
/* we didn't find enough space. See if we can expand the
database and if we can then try again */
if (tdb1_expand(tdb, length + sizeof(*rec)) == 0)
goto again;
fail:
tdb1_unlock(tdb, -1, F_WRLCK);
return 0;
}
