Allow specifying initial and maximum segment sizes for DSA.

Previously, the DSA segment size always started with 1MB and grew up
to DSA_MAX_SEGMENT_SIZE. It was inconvenient in certain scenarios,
such as when the caller desired a soft constraint on the total DSA
segment size, limiting it to less than 1MB.

This commit introduces the capability to specify the initial and
maximum DSA segment sizes when creating a DSA area, providing more
flexibility and control over memory usage.

Reviewed-by: John Naylor, Tomas Vondra
Discussion: https://postgr.es/m/CAD21AoAYGGC1ePjVX0H%2Bpp9rH%3D9vuPK19nNOiu12NprdV5TVJA%40mail.gmail.com

1 files changed, 42 insertions, 4 deletions

diff --git a/src/include/utils/dsa.h b/src/include/utils/dsa.h
index fe9cbebbec1..8dff964bf33 100644
--- a/src/include/utils/dsa.h
+++ b/src/include/utils/dsa.h
@@ -77,6 +77,31 @@ typedef pg_atomic_uint64 dsa_pointer_atomic;
 /* A sentinel value for dsa_pointer used to indicate failure to allocate. */
 #define InvalidDsaPointer ((dsa_pointer) 0)
 
+/*
+ * The number of bits used to represent the offset part of a dsa_pointer.
+ * This controls the maximum size of a segment, the maximum possible
+ * allocation size and also the maximum number of segments per area.
+ */
+#if SIZEOF_DSA_POINTER == 4
+#define DSA_OFFSET_WIDTH 27		/* 32 segments of size up to 128MB */
+#else
+#define DSA_OFFSET_WIDTH 40		/* 1024 segments of size up to 1TB */
+#endif
+
+/*
+ * The default size of the initial DSM segment that backs a dsa_area created
+ * by dsa_create.  After creating some number of segments of the initial size
+ * we'll double this size, and so on.  Larger segments may be created if
+ * necessary to satisfy large requests.
+ */
+#define DSA_DEFAULT_INIT_SEGMENT_SIZE ((size_t) (1 * 1024 * 1024))
+
+/* The minimum size of a DSM segment. */
+#define DSA_MIN_SEGMENT_SIZE	((size_t) (256 * 1024L))
+
+/* The maximum size of a DSM segment. */
+#define DSA_MAX_SEGMENT_SIZE ((size_t) 1 << DSA_OFFSET_WIDTH)
+
 /* Check if a dsa_pointer value is valid. */
 #define DsaPointerIsValid(x) ((x) != InvalidDsaPointer)
 
@@ -88,6 +113,17 @@ typedef pg_atomic_uint64 dsa_pointer_atomic;
 #define dsa_allocate0(area, size) \
 	dsa_allocate_extended(area, size, DSA_ALLOC_ZERO)
 
+/* Create dsa_area with default segment sizes */
+#define dsa_create(tranch_id) \
+	dsa_create_ext(tranch_id, DSA_DEFAULT_INIT_SEGMENT_SIZE, \
+				   DSA_MAX_SEGMENT_SIZE)
+
+/* Create dsa_area with default segment sizes in an existing share memory space */
+#define dsa_create_in_place(place, size, tranch_id, segment) \
+	dsa_create_in_place_ext(place, size, tranch_id, segment, \
+							DSA_DEFAULT_INIT_SEGMENT_SIZE, \
+							DSA_MAX_SEGMENT_SIZE)
+
 /*
  * The type used for dsa_area handles.  dsa_handle values can be shared with
  * other processes, so that they can attach to them.  This provides a way to
@@ -102,10 +138,12 @@ typedef dsm_handle dsa_handle;
 /* Sentinel value to use for invalid dsa_handles. */
 #define DSA_HANDLE_INVALID ((dsa_handle) DSM_HANDLE_INVALID)
 
-
-extern dsa_area *dsa_create(int tranche_id);
-extern dsa_area *dsa_create_in_place(void *place, size_t size,
-									 int tranche_id, dsm_segment *segment);
+extern dsa_area *dsa_create_ext(int tranche_id, size_t init_segment_size,
+								size_t max_segment_size);
+extern dsa_area *dsa_create_in_place_ext(void *place, size_t size,
+										 int tranche_id, dsm_segment *segment,
+										 size_t init_segment_size,
+										 size_t max_segment_size);
 extern dsa_area *dsa_attach(dsa_handle handle);
 extern dsa_area *dsa_attach_in_place(void *place, dsm_segment *segment);
 extern void dsa_release_in_place(void *place);