[5/7,libgomp,amdgcn] Optimize GCN OpenMP malloc performance
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Message ID 31005bba27788173688c0cb8f332c58d27cacbbb.1573560401.git.ams@codesourcery.com
State New
Headers show
Series
  • AMD GCN Offloading Support
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Commit Message

Andrew Stubbs Nov. 12, 2019, 1:29 p.m. UTC
This patch implements a malloc optimization to improve the startup and
shutdown overhead for each OpenMP team.

New malloc functions are created, "team_malloc" and "team_free", that
take memory from a per-team memory arena provided by the plugin, rather
than the shared heap space, which is slow, and gets worse the more teams
are trying to allocate at once.

These new functions are used both in the gcn/team.c file and in selected
places elsewhere in libgomp.  Arena-space is limited (and larger sizes
have greater overhead at launch time) so this should not be a global
search and replace.

Dummy pass-through definitions are provided for other targets.

OK to commit?

Thanks

Andrew


2019-11-12  Andrew Stubbs  <ams@codesourcery.com>

	libgomp/
	* config/gcn/team.c (gomp_gcn_enter_kernel): Set up the team arena
	and use team_malloc variants.
	(gomp_gcn_exit_kernel): Use team_free.
	* libgomp.h (TEAM_ARENA_SIZE): Define.
	(TEAM_ARENA_FREE): Define.
	(TEAM_ARENA_END): Define.
	(team_malloc): New function.
	(team_malloc_cleared): New function.
	(team_free): New function.
	* team.c (gomp_new_team): Use team_malloc.
	(free_team): Use team_free.
	(gomp_free_thread): Use team_free.
	(gomp_pause_host): Use team_free.
	* work.c (gomp_init_work_share): Use team_malloc.
	(gomp_fini_work_share): Use team_free.
---
 libgomp/config/gcn/team.c | 18 ++++++++++---
 libgomp/libgomp.h         | 56 +++++++++++++++++++++++++++++++++++++++
 libgomp/team.c            | 12 ++++-----
 libgomp/work.c            |  4 +--
 4 files changed, 78 insertions(+), 12 deletions(-)

Comments

Jakub Jelinek Nov. 12, 2019, 1:56 p.m. UTC | #1
On Tue, Nov 12, 2019 at 01:29:14PM +0000, Andrew Stubbs wrote:
> 2019-11-12  Andrew Stubbs  <ams@codesourcery.com>
> 
> 	libgomp/
> 	* config/gcn/team.c (gomp_gcn_enter_kernel): Set up the team arena
> 	and use team_malloc variants.
> 	(gomp_gcn_exit_kernel): Use team_free.
> 	* libgomp.h (TEAM_ARENA_SIZE): Define.
> 	(TEAM_ARENA_FREE): Define.
> 	(TEAM_ARENA_END): Define.
> 	(team_malloc): New function.
> 	(team_malloc_cleared): New function.
> 	(team_free): New function.
> 	* team.c (gomp_new_team): Use team_malloc.
> 	(free_team): Use team_free.
> 	(gomp_free_thread): Use team_free.
> 	(gomp_pause_host): Use team_free.
> 	* work.c (gomp_init_work_share): Use team_malloc.
> 	(gomp_fini_work_share): Use team_free.

> +  /* Handle OOM.  */
> +  if (result + size > *(void * __lds *)TEAM_ARENA_END)
> +    {
> +      const char msg[] = "GCN team arena exhausted\n";
> +      write (2, msg, sizeof(msg)-1);
> +      /* It's better to continue with reeduced performance than abort.

s/reeduced/reduced/

Not really sure if it is a good idea to print anything, at least not when
in some debugging mode.  I mean, it is fairly easy to write code that will
trigger this.  And, what is the reason why you can't free the
gomp_malloced memory, like comparing if the team_freed pointer is in between
TEAM_ARENA_START and TEAM_ARENA_END or similar, don't do anything in that
case, otherwise use free?

> +         Beware that this won't get freed, which might cause more problems.  */
> +      result = gomp_malloc (size);
> +    }
> +  return result;
> +}
> +
> +static inline void * __attribute__((malloc)) __attribute__((optimize("-O3")))
> +team_malloc_cleared (size_t size)
> +{
> +  char *result = team_malloc (size);
> +
> +  /* Clear the allocated memory.
> +     This should vectorize.  The allocation has been rounded up to the next
> +     4-byte boundary, so this is safe.  */
> +  for (int i = 0; i<size; i+=4)
> +    *(int*)(result+i) = 0;

Formatting (spaces around <, +=, +, between int and *.  Shouldn't 4 be
sizeof (int)?  And wouldn't memset (result, 0, size); do the same job?

	Jakub
Andrew Stubbs Nov. 12, 2019, 5:47 p.m. UTC | #2
On 12/11/2019 13:56, Jakub Jelinek wrote:
> s/reeduced/reduced/

Done.

> Not really sure if it is a good idea to print anything, at least not when
> in some debugging mode.  I mean, it is fairly easy to write code that will
> trigger this.  And, what is the reason why you can't free the
> gomp_malloced memory, like comparing if the team_freed pointer is in between
> TEAM_ARENA_START and TEAM_ARENA_END or similar, don't do anything in that
> case, otherwise use free?

Falling back to malloc is a big performance hit. There's a global lock 
affecting all teams in all running kernels. If we're running into this 
then a) I want to know about it so I can tune the arena size, and b) I 
want the user to know why performance is suddenly worse.

At least for now, I want to keep the message. I've updated the comment 
though.

>> +  /* Clear the allocated memory.
>> +     This should vectorize.  The allocation has been rounded up to the next
>> +     4-byte boundary, so this is safe.  */
>> +  for (int i = 0; i<size; i+=4)
>> +    *(int*)(result+i) = 0;
> 
> Formatting (spaces around <, +=, +, between int and *.  Shouldn't 4 be
> sizeof (int)?  And wouldn't memset (result, 0, size); do the same job?

I wanted to ensure that the loop would vectorize inline, but I don't 
think it was doing so anyway. I need to look at that, but how is this, 
for now?

Andrew
Jakub Jelinek Nov. 12, 2019, 10:18 p.m. UTC | #3
On Tue, Nov 12, 2019 at 05:47:11PM +0000, Andrew Stubbs wrote:
> > Not really sure if it is a good idea to print anything, at least not when
> > in some debugging mode.  I mean, it is fairly easy to write code that will
> > trigger this.  And, what is the reason why you can't free the
> > gomp_malloced memory, like comparing if the team_freed pointer is in between
> > TEAM_ARENA_START and TEAM_ARENA_END or similar, don't do anything in that
> > case, otherwise use free?
> 
> Falling back to malloc is a big performance hit. There's a global lock
> affecting all teams in all running kernels. If we're running into this then
> a) I want to know about it so I can tune the arena size, and b) I want the
> user to know why performance is suddenly worse.
> 
> At least for now, I want to keep the message. I've updated the comment
> though.

At some point we'll need to pass some data from the host to offloading
target, including ICVs etc. (at least for OpenMP 5.1) and at that point we
can pass also the GOMP_DEBUG state.

> I wanted to ensure that the loop would vectorize inline, but I don't think
> it was doing so anyway. I need to look at that, but how is this, for now?

Ok.

	Jakub

Patch
diff mbox series

diff --git a/libgomp/config/gcn/team.c b/libgomp/config/gcn/team.c
index c566482bda2..063571fc751 100644
--- a/libgomp/config/gcn/team.c
+++ b/libgomp/config/gcn/team.c
@@ -57,16 +57,26 @@  gomp_gcn_enter_kernel (void)
       /* Starting additional threads is not supported.  */
       gomp_global_icv.dyn_var = true;
 
+      /* Initialize the team arena for optimized memory allocation.
+         The arena has been allocated on the host side, and the address
+         passed in via the kernargs.  Each team takes a small slice of it.  */
+      register void **kernargs asm("s8");
+      void *team_arena = (kernargs[4] + TEAM_ARENA_SIZE*teamid);
+      void * __lds *arena_free = (void * __lds *)TEAM_ARENA_FREE;
+      void * __lds *arena_end = (void * __lds *)TEAM_ARENA_END;
+      *arena_free = team_arena;
+      *arena_end = team_arena + TEAM_ARENA_SIZE;
+
       /* Allocate and initialize the team-local-storage data.  */
-      struct gomp_thread *thrs = gomp_malloc_cleared (sizeof (*thrs)
+      struct gomp_thread *thrs = team_malloc_cleared (sizeof (*thrs)
 						      * numthreads);
       set_gcn_thrs (thrs);
 
       /* Allocate and initailize a pool of threads in the team.
          The threads are already running, of course, we just need to manage
          the communication between them.  */
-      struct gomp_thread_pool *pool = gomp_malloc (sizeof (*pool));
-      pool->threads = gomp_malloc (sizeof (void *) * numthreads);
+      struct gomp_thread_pool *pool = team_malloc (sizeof (*pool));
+      pool->threads = team_malloc (sizeof (void *) * numthreads);
       for (int tid = 0; tid < numthreads; tid++)
 	pool->threads[tid] = &thrs[tid];
       pool->threads_size = numthreads;
@@ -91,7 +101,7 @@  void
 gomp_gcn_exit_kernel (void)
 {
   gomp_free_thread (gcn_thrs ());
-  free (gcn_thrs ());
+  team_free (gcn_thrs ());
 }
 
 /* This function contains the idle loop in which a thread waits
diff --git a/libgomp/libgomp.h b/libgomp/libgomp.h
index 19e1241ee4c..659aeb95ffe 100644
--- a/libgomp/libgomp.h
+++ b/libgomp/libgomp.h
@@ -106,6 +106,62 @@  extern void gomp_aligned_free (void *);
    GCC's builtin alloca().  */
 #define gomp_alloca(x)  __builtin_alloca(x)
 
+/* Optimized allocators for team-specific data that will die with the team.  */
+
+#ifdef __AMDGCN__
+/* The arena is initialized in config/gcn/team.c.  */
+#define TEAM_ARENA_SIZE 64*1024  /* Must match the value in plugin-gcn.c.  */
+#define TEAM_ARENA_FREE 16  /* LDS offset of free pointer.  */
+#define TEAM_ARENA_END  24  /* LDS offset of end pointer.  */
+
+static inline void * __attribute__((malloc))
+team_malloc (size_t size)
+{
+  /* 4-byte align the size.  */
+  size = (size + 3) & ~3;
+
+  /* Allocate directly from the arena.
+     The compiler does not support DS atomics, yet. */
+  void *result;
+  asm ("ds_add_rtn_u64 %0, %1, %2\n\ts_waitcnt 0"
+       : "=v"(result) : "v"(TEAM_ARENA_FREE), "v"(size), "e"(1L) : "memory");
+
+  /* Handle OOM.  */
+  if (result + size > *(void * __lds *)TEAM_ARENA_END)
+    {
+      const char msg[] = "GCN team arena exhausted\n";
+      write (2, msg, sizeof(msg)-1);
+      /* It's better to continue with reeduced performance than abort.
+         Beware that this won't get freed, which might cause more problems.  */
+      result = gomp_malloc (size);
+    }
+  return result;
+}
+
+static inline void * __attribute__((malloc)) __attribute__((optimize("-O3")))
+team_malloc_cleared (size_t size)
+{
+  char *result = team_malloc (size);
+
+  /* Clear the allocated memory.
+     This should vectorize.  The allocation has been rounded up to the next
+     4-byte boundary, so this is safe.  */
+  for (int i = 0; i<size; i+=4)
+    *(int*)(result+i) = 0;
+  return result;
+}
+
+static inline void
+team_free (void *ptr)
+{
+  /* The whole arena is freed when the kernel exits.  */
+}
+#else
+#define team_malloc(...) gomp_malloc (__VA_ARGS__)
+#define team_malloc_cleared(...) gomp_malloc_cleared (__VA_ARGS__)
+#define team_free(...) free (__VA_ARGS__)
+#endif
+
 /* error.c */
 
 extern void gomp_vdebug (int, const char *, va_list);
diff --git a/libgomp/team.c b/libgomp/team.c
index b26caaaaec6..cdfb9ba6c98 100644
--- a/libgomp/team.c
+++ b/libgomp/team.c
@@ -171,7 +171,7 @@  gomp_new_team (unsigned nthreads)
     {
       size_t extra = sizeof (team->ordered_release[0])
 		     + sizeof (team->implicit_task[0]);
-      team = gomp_malloc (sizeof (*team) + nthreads * extra);
+      team = team_malloc (sizeof (*team) + nthreads * extra);
 
 #ifndef HAVE_SYNC_BUILTINS
       gomp_mutex_init (&team->work_share_list_free_lock);
@@ -221,7 +221,7 @@  free_team (struct gomp_team *team)
   gomp_barrier_destroy (&team->barrier);
   gomp_mutex_destroy (&team->task_lock);
   priority_queue_free (&team->task_queue);
-  free (team);
+  team_free (team);
 }
 
 static void
@@ -285,8 +285,8 @@  gomp_free_thread (void *arg __attribute__((unused)))
       if (pool->last_team)
 	free_team (pool->last_team);
 #ifndef __nvptx__
-      free (pool->threads);
-      free (pool);
+      team_free (pool->threads);
+      team_free (pool);
 #endif
       thr->thread_pool = NULL;
     }
@@ -1082,8 +1082,8 @@  gomp_pause_host (void)
       if (pool->last_team)
 	free_team (pool->last_team);
 #ifndef __nvptx__
-      free (pool->threads);
-      free (pool);
+      team_free (pool->threads);
+      team_free (pool);
 #endif
       thr->thread_pool = NULL;
     }
diff --git a/libgomp/work.c b/libgomp/work.c
index a589b8b5231..28bb0c11255 100644
--- a/libgomp/work.c
+++ b/libgomp/work.c
@@ -120,7 +120,7 @@  gomp_init_work_share (struct gomp_work_share *ws, size_t ordered,
       else
 	ordered = nthreads * sizeof (*ws->ordered_team_ids);
       if (ordered > INLINE_ORDERED_TEAM_IDS_SIZE)
-	ws->ordered_team_ids = gomp_malloc (ordered);
+	ws->ordered_team_ids = team_malloc (ordered);
       else
 	ws->ordered_team_ids = ws->inline_ordered_team_ids;
       memset (ws->ordered_team_ids, '\0', ordered);
@@ -142,7 +142,7 @@  gomp_fini_work_share (struct gomp_work_share *ws)
 {
   gomp_mutex_destroy (&ws->lock);
   if (ws->ordered_team_ids != ws->inline_ordered_team_ids)
-    free (ws->ordered_team_ids);
+    team_free (ws->ordered_team_ids);
   gomp_ptrlock_destroy (&ws->next_ws);
 }