Increase slab-block size in signal component

The backend allocator for the slab is a sliced heap, which hands out
allocations with page-size granularity (4096 bytes). Therefore, the
slab-block size should also be about a multiple of the page size minus
some bytes of overhead.

Additional adjustments:

- The slab-block size and the default quota-upgrade amount for SIGNAL
  sessions depends on the platform bit width now.
- The signal test also stresses the case of many managed context in one
  session including creation and destruction of the used signal receiver
  in repeated rounds.

repos/base/src/base/signal/signal.cc

@@ -295,8 +295,13 @@ Signal_context_capability Signal_receiver::manage(Signal_context *context)
 			if (try_again)
 				break;
 
-			PINF("upgrading quota donation for SIGNAL session");
-			env()->parent()->upgrade(signal_connection()->cap(), "ram_quota=4K");
+			size_t const quota = 1024*sizeof(long);
+			char buf[64];
+			snprintf(buf, sizeof(buf), "ram_quota=%zu", quota);
+
+			PINF("upgrading quota donation for SIGNAL session (%zu bytes)", quota);
+
+			env()->parent()->upgrade(signal_connection()->cap(), buf);
 			try_again = true;
 		}
 	} while (try_again);

repos/base/src/core/include/signal_session_component.h

@@ -143,7 +143,8 @@ namespace Genode {
 			Signal_source_component               _source;
 			Signal_source_capability              _source_cap;
 			Allocator_guard                       _md_alloc;
-			Tslab<Signal_context_component, 1024> _contexts_slab;
+			Tslab<Signal_context_component,
+			      960*sizeof(long)>               _contexts_slab;
 			Ipc_ostream                          *_ipc_ostream;
 
 		public:

repos/os/src/test/signal/main.cc

@@ -562,6 +562,7 @@ class Signal_context_destroyer : public Thread<4096>
 		}
 };
 
+
 static void synchronized_context_destruction_test()
 {
 	Signal_receiver receiver;
@@ -598,6 +599,25 @@ static void synchronized_context_destruction_test()
 }
 
 
+static void many_managed_contexts()
+{
+	for (unsigned round = 0; round < 10; ++round) {
+
+		unsigned const num_contexts = 200 + 5*round;
+		printf("round %u: create and manage %u contexts\n", round, num_contexts);
+
+		Signal_receiver rec;
+
+		for (unsigned i = 0; i < num_contexts; ++i) {
+			Id_signal_context *context = new (env()->heap()) Id_signal_context(i);
+			rec.manage(context);
+		}
+	}
+
+	printf("many contexts finished\n");
+}
+
+
 /**
  * Main program
  */
@@ -611,6 +631,7 @@ int main(int, char **)
 	lazy_receivers_test();
 	check_context_management();
 	synchronized_context_destruction_test();
+	many_managed_contexts();
 
 	printf("--- signalling test finished ---\n");
 	return 0;