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slab reassignment

Adds a "slabs reassign src dst" manual command, and a thread to safely process
slab moves in the background.

- slab freelist is now a linked list, reusing the item structure
- is -o slab_reassign is enabled, an extra background thread is started
- thread attempts to safely free up items when it's been told to move a page
  from one slab to another.

-o slab_automove is stubbed.

There are some limitations. Most notable is that you cannot repeatedly move
pages around without first having items use up the memory. Slabs with newly
assigned memory work off of a pointer, handing out chunks individually. We
would need to change that to quickly split chunks for all newly assigned pages
into that slabs freelist.

Further testing is required to ensure such is possible without impacting
performance.
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1 parent 40b7b4b commit 10698bae63a034c14d2fdbc3027a1308ce90faba @dormando dormando committed Dec 18, 2011
Showing with 457 additions and 21 deletions.
  1. +2 −0 globals.c
  2. +2 −4 items.c
  3. +68 −1 memcached.c
  4. +19 −0 memcached.h
  5. +284 −15 slabs.c
  6. +10 −0 slabs.h
  7. +1 −1 t/binary.t
  8. +71 −0 t/slabs_reassign.t
View
2 globals.c
@@ -21,3 +21,5 @@ volatile rel_time_t current_time;
/** exported globals **/
struct stats stats;
struct settings settings;
+struct slab_rebalance slab_rebal;
+volatile int slab_rebalance_signal;
View
6 items.c
@@ -123,7 +123,6 @@ item *do_item_alloc(char *key, const size_t nkey, const int flags, const rel_tim
do_item_unlink_nolock(it, hash(ITEM_key(it), it->nkey, 0));
/* Initialize the item block: */
it->slabs_clsid = 0;
- it->refcount = 0;
} else if ((it = slabs_alloc(ntotal, id)) == NULL) {
if (settings.evict_to_free == 0) {
itemstats[id].outofmemory++;
@@ -149,7 +148,6 @@ item *do_item_alloc(char *key, const size_t nkey, const int flags, const rel_tim
do_item_unlink_nolock(it, hash(ITEM_key(it), it->nkey, 0));
/* Initialize the item block: */
it->slabs_clsid = 0;
- it->refcount = 0;
}
} else {
/* If the LRU is empty or locked, attempt to allocate memory */
@@ -181,11 +179,11 @@ item *do_item_alloc(char *key, const size_t nkey, const int flags, const rel_tim
/* Item initialization can happen outside of the lock; the item's already
* been removed from the slab LRU.
*/
+ it->refcount = 1; /* the caller will have a reference */
pthread_mutex_unlock(&cache_lock);
+ it->next = it->prev = it->h_next = 0;
it->slabs_clsid = id;
- it->next = it->prev = it->h_next = 0;
- it->refcount = 1; /* the caller will have a reference */
DEBUG_REFCNT(it, '*');
it->it_flags = settings.use_cas ? ITEM_CAS : 0;
it->nkey = nkey;
View
69 memcached.c
@@ -102,6 +102,9 @@ struct stats stats;
struct settings settings;
time_t process_started; /* when the process was started */
+struct slab_rebalance slab_rebal;
+volatile int slab_rebalance_signal;
+
/** file scope variables **/
static conn *listen_conn = NULL;
static struct event_base *main_base;
@@ -170,7 +173,9 @@ static void stats_init(void) {
stats.curr_bytes = stats.listen_disabled_num = 0;
stats.hash_power_level = stats.hash_bytes = stats.hash_is_expanding = 0;
stats.expired_unfetched = stats.evicted_unfetched = 0;
+ stats.slabs_moved = 0;
stats.accepting_conns = true; /* assuming we start in this state. */
+ stats.slab_reassign_running = false;
/* make the time we started always be 2 seconds before we really
did, so time(0) - time.started is never zero. if so, things
@@ -218,6 +223,8 @@ static void settings_init(void) {
settings.item_size_max = 1024 * 1024; /* The famous 1MB upper limit. */
settings.maxconns_fast = false;
settings.hashpower_init = 0;
+ settings.slab_reassign = false;
+ settings.slab_automove = false;
}
/*
@@ -2572,6 +2579,10 @@ static void server_stats(ADD_STAT add_stats, conn *c) {
APPEND_STAT("hash_is_expanding", "%u", stats.hash_is_expanding);
APPEND_STAT("expired_unfetched", "%llu", stats.expired_unfetched);
APPEND_STAT("evicted_unfetched", "%llu", stats.evicted_unfetched);
+ if (settings.slab_reassign) {
+ APPEND_STAT("slab_reassign_running", "%u", stats.slab_reassign_running);
+ APPEND_STAT("slabs_moved", "%llu", stats.slabs_moved);
+ }
STATS_UNLOCK();
}
@@ -2604,6 +2615,8 @@ static void process_stat_settings(ADD_STAT add_stats, void *c) {
APPEND_STAT("item_size_max", "%d", settings.item_size_max);
APPEND_STAT("maxconns_fast", "%s", settings.maxconns_fast ? "yes" : "no");
APPEND_STAT("hashpower_init", "%d", settings.hashpower_init);
+ APPEND_STAT("slab_reassign", "%s", settings.slab_reassign ? "yes" : "no");
+ APPEND_STAT("slab_automove", "%s", settings.slab_automove ? "yes" : "no");
}
static void process_stat(conn *c, token_t *tokens, const size_t ntokens) {
@@ -3290,6 +3303,45 @@ static void process_command(conn *c, char *command) {
conn_set_state(c, conn_closing);
+ } else if (ntokens == 5 && (strcmp(tokens[COMMAND_TOKEN].value, "slabs") == 0 &&
+ strcmp(tokens[COMMAND_TOKEN + 1].value, "reassign") == 0)) {
+ int src, dst, rv;
+
+ if (settings.slab_reassign == false) {
+ out_string(c, "CLIENT_ERROR slab reassignment disabled");
+ return;
+ }
+
+ src = strtol(tokens[2].value, NULL, 10);
+ dst = strtol(tokens[3].value, NULL, 10);
+
+ if (errno == ERANGE) {
+ out_string(c, "CLIENT_ERROR bad command line format");
+ return;
+ }
+
+ rv = slabs_reassign(src, dst);
+ switch (rv) {
+ case REASSIGN_OK:
+ out_string(c, "OK");
+ break;
+ case REASSIGN_RUNNING:
+ out_string(c, "BUSY");
+ break;
+ case REASSIGN_BADCLASS:
+ out_string(c, "BADCLASS");
+ break;
+ case REASSIGN_NOSPARE:
+ out_string(c, "NOSPARE");
+ break;
+ case REASSIGN_DEST_NOT_FULL:
+ out_string(c, "NOTFULL");
+ break;
+ case REASSIGN_SRC_NOT_SAFE:
+ out_string(c, "UNSAFE");
+ break;
+ }
+ return;
} else if ((ntokens == 3 || ntokens == 4) && (strcmp(tokens[COMMAND_TOKEN].value, "verbosity") == 0)) {
process_verbosity_command(c, tokens, ntokens);
} else {
@@ -4639,11 +4691,15 @@ int main (int argc, char **argv) {
char *subopts_value;
enum {
MAXCONNS_FAST = 0,
- HASHPOWER_INIT
+ HASHPOWER_INIT,
+ SLAB_REASSIGN,
+ SLAB_AUTOMOVE
};
char *const subopts_tokens[] = {
[MAXCONNS_FAST] = "maxconns_fast",
[HASHPOWER_INIT] = "hashpower",
+ [SLAB_REASSIGN] = "slab_reassign",
+ [SLAB_AUTOMOVE] = "slab_automove",
NULL
};
@@ -4889,6 +4945,12 @@ int main (int argc, char **argv) {
return 1;
}
break;
+ case SLAB_REASSIGN:
+ settings.slab_reassign = true;
+ break;
+ case SLAB_AUTOMOVE:
+ settings.slab_automove = true;
+ break;
default:
printf("Illegal suboption \"%s\"\n", subopts_value);
return 1;
@@ -5042,6 +5104,11 @@ int main (int argc, char **argv) {
exit(EXIT_FAILURE);
}
+ if (settings.slab_reassign &&
+ start_slab_maintenance_thread() == -1) {
+ exit(EXIT_FAILURE);
+ }
+
/* initialise clock event */
clock_handler(0, 0, 0);
View
19 memcached.h
@@ -265,6 +265,8 @@ struct stats {
bool hash_is_expanding; /* If the hash table is being expanded */
uint64_t expired_unfetched; /* items reclaimed but never touched */
uint64_t evicted_unfetched; /* items evicted but never touched */
+ bool slab_reassign_running; /* slab reassign in progress */
+ uint64_t slabs_moved; /* times slabs were moved around */
};
#define MAX_VERBOSITY_LEVEL 2
@@ -298,6 +300,8 @@ struct settings {
int item_size_max; /* Maximum item size, and upper end for slabs */
bool sasl; /* SASL on/off */
bool maxconns_fast; /* Whether or not to early close connections */
+ bool slab_reassign; /* Whether or not slab reassignment is allowed */
+ bool slab_automove; /* Whether or not to automatically move slabs */
int hashpower_init; /* Starting hash power level */
};
@@ -452,6 +456,21 @@ struct conn {
/* current time of day (updated periodically) */
extern volatile rel_time_t current_time;
+/* TODO: Move to slabs.h? */
+extern volatile int slab_rebalance_signal;
+
+struct slab_rebalance {
+ void *slab_start;
+ void *slab_end;
+ void *slab_pos;
+ int s_clsid;
+ int d_clsid;
+ int busy_items;
+ uint8_t done;
+};
+
+extern struct slab_rebalance slab_rebal;
+
/*
* Functions
*/
View
299 slabs.c
@@ -27,9 +27,8 @@ typedef struct {
unsigned int size; /* sizes of items */
unsigned int perslab; /* how many items per slab */
- void **slots; /* list of item ptrs */
- unsigned int sl_total; /* size of previous array */
- unsigned int sl_curr; /* first free slot */
+ void *slots; /* list of item ptrs */
+ unsigned int sl_curr; /* total free items in list */
void *end_page_ptr; /* pointer to next free item at end of page, or 0 */
unsigned int end_page_free; /* number of items remaining at end of last alloced page */
@@ -192,7 +191,8 @@ static int grow_slab_list (const unsigned int id) {
static int do_slabs_newslab(const unsigned int id) {
slabclass_t *p = &slabclass[id];
- int len = p->size * p->perslab;
+ int len = settings.slab_reassign ? settings.item_size_max
+ : p->size * p->perslab;
char *ptr;
if ((mem_limit && mem_malloced + len > mem_limit && p->slabs > 0) ||
@@ -218,14 +218,15 @@ static int do_slabs_newslab(const unsigned int id) {
static void *do_slabs_alloc(const size_t size, unsigned int id) {
slabclass_t *p;
void *ret = NULL;
+ item *it = NULL;
if (id < POWER_SMALLEST || id > power_largest) {
MEMCACHED_SLABS_ALLOCATE_FAILED(size, 0);
return NULL;
}
p = &slabclass[id];
- assert(p->sl_curr == 0 || ((item *)p->slots[p->sl_curr - 1])->slabs_clsid == 0);
+ assert(p->sl_curr == 0 || ((item *)p->slots)->slabs_clsid == 0);
#ifdef USE_SYSTEM_MALLOC
if (mem_limit && mem_malloced + size > mem_limit) {
@@ -246,7 +247,10 @@ static void *do_slabs_alloc(const size_t size, unsigned int id) {
ret = NULL;
} else if (p->sl_curr != 0) {
/* return off our freelist */
- ret = p->slots[--p->sl_curr];
+ it = (item *)p->slots;
+ p->slots = it->next;
+ p->sl_curr--;
+ ret = (void *)it;
} else {
/* if we recently allocated a whole page, return from that */
assert(p->end_page_ptr != NULL);
@@ -270,6 +274,7 @@ static void *do_slabs_alloc(const size_t size, unsigned int id) {
static void do_slabs_free(void *ptr, const size_t size, unsigned int id) {
slabclass_t *p;
+ item *it;
assert(((item *)ptr)->slabs_clsid == 0);
assert(id >= POWER_SMALLEST && id <= power_largest);
@@ -285,15 +290,13 @@ static void do_slabs_free(void *ptr, const size_t size, unsigned int id) {
return;
#endif
- if (p->sl_curr == p->sl_total) { /* need more space on the free list */
- int new_size = (p->sl_total != 0) ? p->sl_total * 2 : 16; /* 16 is arbitrary */
- void **new_slots = realloc(p->slots, new_size * sizeof(void *));
- if (new_slots == 0)
- return;
- p->slots = new_slots;
- p->sl_total = new_size;
- }
- p->slots[p->sl_curr++] = ptr;
+ it = (item *)ptr;
+ it->prev = 0;
+ it->next = p->slots;
+ if (it->next) it->next->prev = it;
+ p->slots = it;
+
+ p->sl_curr++;
p->requested -= size;
return;
}
@@ -453,3 +456,269 @@ void slabs_adjust_mem_requested(unsigned int id, size_t old, size_t ntotal)
p->requested = p->requested - old + ntotal;
pthread_mutex_unlock(&slabs_lock);
}
+
+static pthread_cond_t maintenance_cond = PTHREAD_COND_INITIALIZER;
+static volatile int do_run_slab_thread = 1;
+
+#define DEFAULT_SLAB_BULK_CHECK 1
+int slab_bulk_check = DEFAULT_SLAB_BULK_CHECK;
+
+static int slab_rebalance_start(void) {
+ slabclass_t *s_cls;
+ slabclass_t *d_cls;
+ int no_go = 0;
+
+ pthread_mutex_lock(&cache_lock);
+ pthread_mutex_lock(&slabs_lock);
+
+ if (slab_rebal.s_clsid < POWER_SMALLEST ||
+ slab_rebal.s_clsid > power_largest ||
+ slab_rebal.d_clsid < POWER_SMALLEST ||
+ slab_rebal.d_clsid > power_largest)
+ no_go = -2;
+
+ s_cls = &slabclass[slab_rebal.s_clsid];
+ d_cls = &slabclass[slab_rebal.d_clsid];
+
+ if (d_cls->end_page_ptr || s_cls->end_page_ptr ||
+ !grow_slab_list(slab_rebal.d_clsid)) {
+ no_go = -1;
+ }
+
+ if (s_cls->slabs < 2)
+ no_go = -3;
+
+ if (no_go != 0) {
+ pthread_mutex_unlock(&slabs_lock);
+ pthread_mutex_unlock(&cache_lock);
+ return no_go; /* Should use a wrapper function... */
+ }
+
+ s_cls->killing = 1;
+
+ slab_rebal.slab_start = s_cls->slab_list[s_cls->killing - 1];
+ slab_rebal.slab_end = (char *)slab_rebal.slab_start +
+ (s_cls->size * s_cls->perslab);
+ slab_rebal.slab_pos = slab_rebal.slab_start;
+ slab_rebal.done = 0;
+
+ /* Also tells do_item_get to search for items in this slab */
+ slab_rebalance_signal = 2;
+
+ if (settings.verbose > 1) {
+ fprintf(stderr, "Started a slab rebalance\n");
+ }
+
+ pthread_mutex_unlock(&slabs_lock);
+ pthread_mutex_unlock(&cache_lock);
+
+ STATS_LOCK();
+ stats.slab_reassign_running = true;
+ STATS_UNLOCK();
+
+ return 0;
+}
+
+/* refcount == 0 is safe since nobody can incr while cache_lock is held.
+ * refcount != 0 is impossible since flags/etc can be modified in other
+ * threads. instead, note we found a busy one and bail. logic in do_item_get
+ * will prevent busy items from continuing to be busy
+ */
+static int slab_rebalance_move(void) {
+ slabclass_t *s_cls;
+ int x;
+ int was_busy = 0;
+
+ pthread_mutex_lock(&cache_lock);
+ pthread_mutex_lock(&slabs_lock);
+
+ s_cls = &slabclass[slab_rebal.s_clsid];
+
+ for (x = 0; x < slab_bulk_check; x++) {
+ item *it = slab_rebal.slab_pos;
+ if (it->refcount == 0) {
+ if (it->it_flags & ITEM_SLABBED) {
+ /* remove from freelist linked list */
+ if (s_cls->slots == it) {
+ s_cls->slots = it->next;
+ }
+ if (it->next) it->next->prev = it->prev;
+ if (it->prev) it->prev->next = it->next;
+ s_cls->sl_curr--;
+ } else if (it->it_flags != 0) {
+ it->refcount = 1;
+ /* Call unlink with refcount == 1 so it won't free */
+ do_item_unlink_nolock(it, hash(ITEM_key(it), it->nkey, 0));
+ it->refcount = 0;
+ }
+ it->it_flags = 0;
+ it->slabs_clsid = 0;
+ } else {
+ if (settings.verbose > 2) {
+ fprintf(stderr, "Slab reassign hit a busy item: refcount: %d (%d -> %d)\n",
+ it->refcount, slab_rebal.s_clsid, slab_rebal.d_clsid);
+ }
+ slab_rebal.busy_items++;
+ was_busy++;
+ }
+
+ slab_rebal.slab_pos = (char *)slab_rebal.slab_pos + s_cls->size;
+ if (slab_rebal.slab_pos >= slab_rebal.slab_end)
+ break;
+ }
+
+ if (slab_rebal.slab_pos >= slab_rebal.slab_end) {
+ /* Some items were busy, start again from the top */
+ if (slab_rebal.busy_items) {
+ slab_rebal.slab_pos = slab_rebal.slab_start;
+ slab_rebal.busy_items = 0;
+ } else {
+ slab_rebal.done++;
+ }
+ }
+
+ pthread_mutex_unlock(&slabs_lock);
+ pthread_mutex_unlock(&cache_lock);
+
+ return was_busy;
+}
+
+static void slab_rebalance_finish(void) {
+ slabclass_t *s_cls;
+ slabclass_t *d_cls;
+
+ pthread_mutex_lock(&cache_lock);
+ pthread_mutex_lock(&slabs_lock);
+
+ s_cls = &slabclass[slab_rebal.s_clsid];
+ d_cls = &slabclass[slab_rebal.d_clsid];
+
+ /* At this point the stolen slab is completely clear */
+ s_cls->slab_list[s_cls->killing - 1] =
+ s_cls->slab_list[s_cls->slabs - 1];
+ s_cls->slabs--;
+ s_cls->killing = 0;
+
+ memset(slab_rebal.slab_start, 0, (size_t)settings.item_size_max);
+
+ d_cls->slab_list[d_cls->slabs++] = slab_rebal.slab_start;
+ d_cls->end_page_ptr = slab_rebal.slab_start;
+ d_cls->end_page_free = d_cls->perslab;
+
+ slab_rebal.done = 0;
+ slab_rebal.s_clsid = 0;
+ slab_rebal.d_clsid = 0;
+ slab_rebal.slab_start = NULL;
+ slab_rebal.slab_end = NULL;
+ slab_rebal.slab_pos = NULL;
+
+ slab_rebalance_signal = 0;
+
+ pthread_mutex_unlock(&slabs_lock);
+ pthread_mutex_unlock(&cache_lock);
+
+ STATS_LOCK();
+ stats.slab_reassign_running = false;
+ stats.slabs_moved++;
+ STATS_UNLOCK();
+
+ if (settings.verbose > 1) {
+ fprintf(stderr, "finished a slab move\n");
+ }
+}
+
+/* Slab rebalancer thread.
+ * Does not use spinlocks since it is not timing sensitive. Burn less CPU and
+ * go to sleep if locks are contended
+ */
+static void *slab_maintenance_thread(void *arg) {
+ int was_busy = 0;
+
+ while (do_run_slab_thread) {
+ /* TODO: Call code to make a calculated decision */
+
+ if (slab_rebalance_signal == 1) {
+ if (slab_rebalance_start() < 0) {
+ /* Handle errors with more specifity as required. */
+ slab_rebalance_signal = 0;
+ }
+
+ } else if (slab_rebalance_signal && slab_rebal.slab_start != NULL) {
+ /* If we have a decision to continue, continue it */
+ was_busy = slab_rebalance_move();
+ }
+
+ if (slab_rebal.done) {
+ slab_rebalance_finish();
+ }
+
+ /* Sleep a bit if no work to do, or waiting on busy objects */
+ if (was_busy || !slab_rebalance_signal)
+ sleep(1);
+ }
+ return NULL;
+}
+
+static enum reassign_result_type do_slabs_reassign(int src, int dst) {
+ if (slab_rebalance_signal != 0)
+ return REASSIGN_RUNNING;
+
+ if (src < POWER_SMALLEST || src > power_largest ||
+ dst < POWER_SMALLEST || dst > power_largest)
+ return REASSIGN_BADCLASS;
+
+ if (slabclass[src].slabs < 2)
+ return REASSIGN_NOSPARE;
+
+ if (slabclass[dst].end_page_ptr)
+ return REASSIGN_DEST_NOT_FULL;
+
+ if (slabclass[src].end_page_ptr)
+ return REASSIGN_SRC_NOT_SAFE;
+
+ slab_rebal.s_clsid = src;
+ slab_rebal.d_clsid = dst;
+
+ slab_rebalance_signal = 1;
+
+ return REASSIGN_OK;
+}
+
+enum reassign_result_type slabs_reassign(int src, int dst) {
+ enum reassign_result_type ret;
+ mutex_lock(&slabs_lock);
+ ret = do_slabs_reassign(src, dst);
+ pthread_mutex_unlock(&slabs_lock);
+ return ret;
+}
+
+static pthread_t maintenance_tid;
+
+int start_slab_maintenance_thread(void) {
+ int ret;
+ slab_rebalance_signal = 0;
+ slab_rebal.slab_start = NULL;
+ char *env = getenv("MEMCACHED_SLAB_BULK_CHECK");
+ if (env != NULL) {
+ slab_bulk_check = atoi(env);
+ if (slab_bulk_check == 0) {
+ slab_bulk_check = DEFAULT_SLAB_BULK_CHECK;
+ }
+ }
+ if ((ret = pthread_create(&maintenance_tid, NULL,
+ slab_maintenance_thread, NULL)) != 0) {
+ fprintf(stderr, "Can't create thread: %s\n", strerror(ret));
+ return -1;
+ }
+ return 0;
+}
+
+void stop_slab_maintenance_thread(void) {
+ mutex_lock(&cache_lock);
+ do_run_slab_thread = 0;
+ pthread_cond_signal(&maintenance_cond);
+ pthread_mutex_unlock(&cache_lock);
+
+ /* Wait for the maintenance thread to stop */
+ pthread_join(maintenance_tid, NULL);
+}
View
10 slabs.h
@@ -33,4 +33,14 @@ bool get_stats(const char *stat_type, int nkey, ADD_STAT add_stats, void *c);
/** Fill buffer with stats */ /*@null@*/
void slabs_stats(ADD_STAT add_stats, void *c);
+int start_slab_maintenance_thread(void);
+void stop_slab_maintenance_thread(void);
+
+enum reassign_result_type {
+ REASSIGN_OK=0, REASSIGN_RUNNING, REASSIGN_BADCLASS, REASSIGN_NOSPARE,
+ REASSIGN_DEST_NOT_FULL, REASSIGN_SRC_NOT_SAFE
+};
+
+enum reassign_result_type slabs_reassign(int src, int dst);
+
#endif
View
2 t/binary.t
@@ -2,7 +2,7 @@
use strict;
use warnings;
-use Test::More tests => 3533;
+use Test::More tests => 3539;
use FindBin qw($Bin);
use lib "$Bin/lib";
use MemcachedTest;
View
71 t/slabs_reassign.t
@@ -0,0 +1,71 @@
+#!/usr/bin/perl
+
+use strict;
+use warnings;
+use Test::More tests => 111;
+use FindBin qw($Bin);
+use lib "$Bin/lib";
+use MemcachedTest;
+
+# Enable manual slab reassign, cap at 6 slabs
+my $server = new_memcached('-o slab_reassign -m 4');
+my $stats = mem_stats($server->sock, ' settings');
+is($stats->{slab_reassign}, "yes");
+
+my $sock = $server->sock;
+
+# Fill a largeish slab until it evicts (honors the -m 6)
+my $bigdata = 'x' x 70000; # slab 31
+for (1 .. 50) {
+ print $sock "set bfoo$_ 0 0 70000\r\n", $bigdata, "\r\n";
+ is(scalar <$sock>, "STORED\r\n", "stored key");
+}
+
+# Fill a smaller slab until it evicts
+my $smalldata = 'y' x 20000; # slab 25
+for (1 .. 50) {
+ print $sock "set sfoo$_ 0 0 20000\r\n", $smalldata, "\r\n";
+ is(scalar <$sock>, "STORED\r\n", "stored key");
+}
+
+my $items_before = mem_stats($sock, "items");
+isnt($items_before->{"items:31:evicted"}, 0, "slab 31 evicted is nonzero");
+isnt($items_before->{"items:25:evicted"}, 0, "slab 25 evicted is nonzero");
+
+my $slabs_before = mem_stats($sock, "slabs");
+# Move a large slab to the smaller slab
+print $sock "slabs reassign 31 25\r\n";
+is(scalar <$sock>, "OK\r\n", "slab rebalancer started");
+
+# Still working out how/if to signal the thread. For now, just sleep.
+sleep 2;
+
+# Check that stats counters increased
+my $slabs_after = mem_stats($sock, "slabs");
+my $stats = mem_stats($sock);
+
+isnt($stats->{slabs_moved}, 0, "slabs moved is nonzero");
+
+# Check that slab stats reflect the change
+ok($slabs_before->{"31:total_pages"} != $slabs_after->{"31:total_pages"},
+ "slab 31 pagecount changed");
+ok($slabs_before->{"25:total_pages"} != $slabs_after->{"25:total_pages"},
+ "slab 25 pagecount changed");
+
+# Try to move another slab, see that it complains
+print $sock "slabs reassign 31 25\r\n";
+is(scalar <$sock>, "NOTFULL\r\n", "Cannot re-run against class with empty space");
+
+# Try to move a page backwards. Should complain that source class isn't "safe"
+# to move from.
+print $sock "slabs reassign 25 31\r\n";
+is(scalar <$sock>, "UNSAFE\r\n", "Cannot move an unsafe slab back");
+
+# Try to insert items into both slabs
+print $sock "set bfoo51 0 0 70000\r\n", $bigdata, "\r\n";
+is(scalar <$sock>, "STORED\r\n", "stored key");
+
+print $sock "set sfoo51 0 0 20000\r\n", $smalldata, "\r\n";
+is(scalar <$sock>, "STORED\r\n", "stored key");
+
+# Do need to come up with better automated tests for this.

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