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Commitdaa7527

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Reduce the number of semaphores used under --disable-spinlocks.
Instead of allocating a semaphore from the operating system for everyspinlock, allocate a fixed number of semaphores (by default, 1024)from the operating system and multiplex all the spinlocks that getcreated onto them. This could self-deadlock if a process attemptedto acquire more than one spinlock at a time, but since processesaren't supposed to execute anything other than short stretches ofstraight-line code while holding a spinlock, that shouldn't happen.One motivation for this change is that, with the introduction ofdynamic shared memory, it may be desirable to create spinlocks thatlast for less than the lifetime of the server. Without this change,attempting to use such facilities under --disable-spinlocks wouldquickly exhaust any supply of available semaphores. Quite apartfrom that, it's desirable to contain the quantity of semaphoresneeded to run the server simply on convenience grounds, since usingtoo many may make it harder to get PostgreSQL running on a newplatform, which is mostly the point of --disable-spinlocks in thefirst place.Patch by me; review by Tom Lane.
1 parent3739e5a commitdaa7527

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7 files changed

+74
-23
lines changed

7 files changed

+74
-23
lines changed

‎src/backend/postmaster/postmaster.c

Lines changed: 9 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -471,6 +471,9 @@ typedef struct
471471
slock_t*ShmemLock;
472472
VariableCacheShmemVariableCache;
473473
Backend*ShmemBackendArray;
474+
#ifndefHAVE_SPINLOCKS
475+
PGSemaphoreSpinlockSemaArray;
476+
#endif
474477
LWLock*LWLockArray;
475478
slock_t*ProcStructLock;
476479
PROC_HDR*ProcGlobal;
@@ -5626,6 +5629,9 @@ save_backend_variables(BackendParameters *param, Port *port,
56265629
param->ShmemVariableCache=ShmemVariableCache;
56275630
param->ShmemBackendArray=ShmemBackendArray;
56285631

5632+
#ifndefHAVE_SPINLOCKS
5633+
param->SpinlockSemaArray=SpinlockSemaArray;
5634+
#endif
56295635
param->LWLockArray=LWLockArray;
56305636
param->ProcStructLock=ProcStructLock;
56315637
param->ProcGlobal=ProcGlobal;
@@ -5854,6 +5860,9 @@ restore_backend_variables(BackendParameters *param, Port *port)
58545860
ShmemVariableCache=param->ShmemVariableCache;
58555861
ShmemBackendArray=param->ShmemBackendArray;
58565862

5863+
#ifndefHAVE_SPINLOCKS
5864+
SpinlockSemaArray=param->SpinlockSemaArray;
5865+
#endif
58575866
LWLockArray=param->LWLockArray;
58585867
ProcStructLock=param->ProcStructLock;
58595868
ProcGlobal=param->ProcGlobal;

‎src/backend/storage/ipc/ipci.c

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -105,6 +105,7 @@ CreateSharedMemoryAndSemaphores(bool makePrivate, int port)
105105
* need to be so careful during the actual allocation phase.
106106
*/
107107
size=100000;
108+
size=add_size(size,SpinlockSemaSize());
108109
size=add_size(size,hash_estimate_size(SHMEM_INDEX_SIZE,
109110
sizeof(ShmemIndexEnt)));
110111
size=add_size(size,BufferShmemSize());

‎src/backend/storage/ipc/shmem.c

Lines changed: 18 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -116,9 +116,24 @@ InitShmemAllocation(void)
116116
Assert(shmhdr!=NULL);
117117

118118
/*
119-
* Initialize the spinlock used by ShmemAlloc.We have to do the space
120-
* allocation the hard way, since obviously ShmemAlloc can't be called
121-
* yet.
119+
* If spinlocks are disabled, initialize emulation layer. We have to do
120+
* the space allocation the hard way, since obviously ShmemAlloc can't be
121+
* called yet.
122+
*/
123+
#ifndefHAVE_SPINLOCKS
124+
{
125+
PGSemaphorespinsemas;
126+
127+
spinsemas= (PGSemaphore) (((char*)shmhdr)+shmhdr->freeoffset);
128+
shmhdr->freeoffset+=MAXALIGN(SpinlockSemaSize());
129+
SpinlockSemaInit(spinsemas);
130+
Assert(shmhdr->freeoffset <=shmhdr->totalsize);
131+
}
132+
#endif
133+
134+
/*
135+
* Initialize the spinlock used by ShmemAlloc; we have to do this the hard
136+
* way, too, for the same reasons as above.
122137
*/
123138
ShmemLock= (slock_t*) (((char*)shmhdr)+shmhdr->freeoffset);
124139
shmhdr->freeoffset+=MAXALIGN(sizeof(slock_t));

‎src/backend/storage/lmgr/spin.c

Lines changed: 31 additions & 19 deletions
Original file line numberDiff line numberDiff line change
@@ -29,6 +29,18 @@
2929
#include"storage/spin.h"
3030

3131

32+
PGSemaphoreSpinlockSemaArray;
33+
34+
/*
35+
* Report the amount of shared memory needed to store semaphores for spinlock
36+
* support.
37+
*/
38+
Size
39+
SpinlockSemaSize(void)
40+
{
41+
returnSpinlockSemas()*sizeof(PGSemaphoreData);
42+
}
43+
3244
#ifdefHAVE_SPINLOCKS
3345

3446
/*
@@ -52,22 +64,20 @@ SpinlockSemas(void)
5264
int
5365
SpinlockSemas(void)
5466
{
55-
intnsemas;
56-
57-
/*
58-
* It would be cleaner to distribute this logic into the affected modules,
59-
* similar to the way shmem space estimation is handled.
60-
*
61-
* For now, though, there are few enough users of spinlocks that we just
62-
* keep the knowledge here.
63-
*/
64-
nsemas=NumLWLocks();/* one for each lwlock */
65-
nsemas+=NBuffers;/* one for each buffer header */
66-
nsemas+=max_wal_senders;/* one for each wal sender process */
67-
nsemas+=num_xloginsert_slots;/* one for each WAL insertion slot */
68-
nsemas+=30;/* plus a bunch for other small-scale use */
69-
70-
returnnsemas;
67+
returnNUM_SPINLOCK_SEMAPHORES;
68+
}
69+
70+
/*
71+
* Initialize semaphores.
72+
*/
73+
externvoid
74+
SpinlockSemaInit(PGSemaphorespinsemas)
75+
{
76+
inti;
77+
78+
for (i=0;i<NUM_SPINLOCK_SEMAPHORES;++i)
79+
PGSemaphoreCreate(&spinsemas[i]);
80+
SpinlockSemaArray=spinsemas;
7181
}
7282

7383
/*
@@ -77,13 +87,15 @@ SpinlockSemas(void)
7787
void
7888
s_init_lock_sema(volatileslock_t*lock)
7989
{
80-
PGSemaphoreCreate((PGSemaphore)lock);
90+
staticintcounter=0;
91+
92+
*lock= (++counter) %NUM_SPINLOCK_SEMAPHORES;
8193
}
8294

8395
void
8496
s_unlock_sema(volatileslock_t*lock)
8597
{
86-
PGSemaphoreUnlock((PGSemaphore)lock);
98+
PGSemaphoreUnlock(&SpinlockSemaArray[*lock]);
8799
}
88100

89101
bool
@@ -98,7 +110,7 @@ int
98110
tas_sema(volatileslock_t*lock)
99111
{
100112
/* Note that TAS macros return 0 if *success* */
101-
return !PGSemaphoreTryLock((PGSemaphore)lock);
113+
return !PGSemaphoreTryLock(&SpinlockSemaArray[*lock]);
102114
}
103115

104116
#endif/* !HAVE_SPINLOCKS */

‎src/include/pg_config_manual.h

Lines changed: 8 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -56,6 +56,14 @@
5656
*/
5757
#defineNUM_USER_DEFINED_LWLOCKS4
5858

59+
/*
60+
* When we don't have native spinlocks, we use semaphores to simulate them.
61+
* Decreasing this value reduces consumption of OS resources; increasing it
62+
* may improve performance, but supplying a real spinlock implementation is
63+
* probably far better.
64+
*/
65+
#defineNUM_SPINLOCK_SEMAPHORES1024
66+
5967
/*
6068
* Define this if you want to allow the lo_import and lo_export SQL
6169
* functions to be executed by ordinary users.By default these

‎src/include/storage/s_lock.h

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -915,7 +915,7 @@ spin_delay(void)
915915
* to fall foul of kernel limits on number of semaphores, so don't use this
916916
* unless you must! The subroutines appear in spin.c.
917917
*/
918-
typedefPGSemaphoreDataslock_t;
918+
typedefintslock_t;
919919

920920
externbools_lock_free_sema(volatileslock_t*lock);
921921
externvoids_unlock_sema(volatileslock_t*lock);

‎src/include/storage/spin.h

Lines changed: 6 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -69,5 +69,11 @@
6969

7070

7171
externintSpinlockSemas(void);
72+
externSizeSpinlockSemaSize(void);
73+
74+
#ifndefHAVE_SPINLOCKS
75+
externvoidSpinlockSemaInit(PGSemaphore);
76+
externPGSemaphoreSpinlockSemaArray;
77+
#endif
7278

7379
#endif/* SPIN_H */

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