Simplify the effective_io_concurrency setting.

The effective_io_concurrency GUC and equivalent tablespace option were
previously passed through a formula based on a theory about RAID
spindles and probabilities, to arrive at the number of pages to prefetch
in bitmap heap scans.  Tomas Vondra, Andres Freund and others argued
that it was anachronistic and hard to justify, and commit 558a9165e08
already started down the path of bypassing it in new code.  We agreed to
drop that logic and use the value directly.

For the default setting of 1, there is no change in effect.  Higher
settings can be converted from the old meaning to the new with:

  select round(sum(OLD / n::float)) from generate_series(1, OLD) s(n);

We might want to consider renaming the GUC before the next release given
the change in meaning, but it's not clear that many users had set it
very carefully anyway.  That decision is deferred for now.

Discussion: https://postgr.es/m/CA%2BhUKGJUw08dPs_3EUcdO6M90GnjofPYrWp4YSLaBkgYwS-AqA%40mail.gmail.com

diff --git a/src/backend/executor/nodeBitmapHeapscan.c b/src/backend/executor/nodeBitmapHeapscan.c
index ae8a11da304d1d9fb189d6bf4cbb38627b4d035d..726d3a2d9a4272afdda8229914dcc79bc76315cb 100644 (file)
--- a/src/backend/executor/nodeBitmapHeapscan.c
+++ b/src/backend/executor/nodeBitmapHeapscan.c
@@ -707,7 +707,6 @@ ExecInitBitmapHeapScan(BitmapHeapScan *node, EState *estate, int eflags)
 {
    BitmapHeapScanState *scanstate;
    Relation    currentRelation;
-   int         io_concurrency;
 
    /* check for unsupported flags */
    Assert(!(eflags & (EXEC_FLAG_BACKWARD | EXEC_FLAG_MARK)));
@@ -737,8 +736,6 @@ ExecInitBitmapHeapScan(BitmapHeapScan *node, EState *estate, int eflags)
    scanstate->prefetch_iterator = NULL;
    scanstate->prefetch_pages = 0;
    scanstate->prefetch_target = 0;
-   /* may be updated below */
-   scanstate->prefetch_maximum = target_prefetch_pages;
    scanstate->pscan_len = 0;
    scanstate->initialized = false;
    scanstate->shared_tbmiterator = NULL;
@@ -794,20 +791,11 @@ ExecInitBitmapHeapScan(BitmapHeapScan *node, EState *estate, int eflags)
        ExecInitQual(node->bitmapqualorig, (PlanState *) scanstate);
 
    /*
-    * Determine the maximum for prefetch_target.  If the tablespace has a
-    * specific IO concurrency set, use that to compute the corresponding
-    * maximum value; otherwise, we already initialized to the value computed
-    * by the GUC machinery.
+    * Maximum number of prefetches for the tablespace if configured, otherwise
+    * the current value of the effective_io_concurrency GUC.
     */
-   io_concurrency =
+   scanstate->prefetch_maximum =
        get_tablespace_io_concurrency(currentRelation->rd_rel->reltablespace);
-   if (io_concurrency != effective_io_concurrency)
-   {
-       double      maximum;
-
-       if (ComputeIoConcurrency(io_concurrency, &maximum))
-           scanstate->prefetch_maximum = rint(maximum);
-   }
 
    scanstate->ss.ss_currentRelation = currentRelation;
 

diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 588005424547a8043818f7ad043ab7ba0b173077..7a7748b6955f957ba62bd9c50a86b1e6c436589b 100644 (file)
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -110,6 +110,13 @@ bool       zero_damaged_pages = false;
 int            bgwriter_lru_maxpages = 100;
 double     bgwriter_lru_multiplier = 2.0;
 bool       track_io_timing = false;
+
+/*
+ * How many buffers PrefetchBuffer callers should try to stay ahead of their
+ * ReadBuffer calls by.  Zero means "never prefetch".  This value is only used
+ * for buffers not belonging to tablespaces that have their
+ * effective_io_concurrency parameter set.
+ */
 int            effective_io_concurrency = 0;
 
 /*
@@ -120,15 +127,6 @@ int            checkpoint_flush_after = 0;
 int            bgwriter_flush_after = 0;
 int            backend_flush_after = 0;
 
-/*
- * How many buffers PrefetchBuffer callers should try to stay ahead of their
- * ReadBuffer calls by.  This is maintained by the assign hook for
- * effective_io_concurrency.  Zero means "never prefetch".  This value is
- * only used for buffers not belonging to tablespaces that have their
- * effective_io_concurrency parameter set.
- */
-int            target_prefetch_pages = 0;
-
 /* local state for StartBufferIO and related functions */
 static BufferDesc *InProgressBuf = NULL;
 static bool IsForInput;
@@ -461,64 +459,6 @@ static int ckpt_buforder_comparator(const void *pa, const void *pb);
 static int ts_ckpt_progress_comparator(Datum a, Datum b, void *arg);
 
 
-/*
- * ComputeIoConcurrency -- get the number of pages to prefetch for a given
- *     number of spindles.
- */
-bool
-ComputeIoConcurrency(int io_concurrency, double *target)
-{
-   double      new_prefetch_pages = 0.0;
-   int         i;
-
-   /*
-    * Make sure the io_concurrency value is within valid range; it may have
-    * been forced with a manual pg_tablespace update.
-    */
-   io_concurrency = Min(Max(io_concurrency, 0), MAX_IO_CONCURRENCY);
-
-   /*----------
-    * The user-visible GUC parameter is the number of drives (spindles),
-    * which we need to translate to a number-of-pages-to-prefetch target.
-    * The target value is stashed in *extra and then assigned to the actual
-    * variable by assign_effective_io_concurrency.
-    *
-    * The expected number of prefetch pages needed to keep N drives busy is:
-    *
-    * drives |   I/O requests
-    * -------+----------------
-    *      1 |   1
-    *      2 |   2/1 + 2/2 = 3
-    *      3 |   3/1 + 3/2 + 3/3 = 5 1/2
-    *      4 |   4/1 + 4/2 + 4/3 + 4/4 = 8 1/3
-    *      n |   n * H(n)
-    *
-    * This is called the "coupon collector problem" and H(n) is called the
-    * harmonic series.  This could be approximated by n * ln(n), but for
-    * reasonable numbers of drives we might as well just compute the series.
-    *
-    * Alternatively we could set the target to the number of pages necessary
-    * so that the expected number of active spindles is some arbitrary
-    * percentage of the total.  This sounds the same but is actually slightly
-    * different.  The result ends up being ln(1-P)/ln((n-1)/n) where P is
-    * that desired fraction.
-    *
-    * Experimental results show that both of these formulas aren't aggressive
-    * enough, but we don't really have any better proposals.
-    *
-    * Note that if io_concurrency = 0 (disabled), we must set target = 0.
-    *----------
-    */
-
-   for (i = 1; i <= io_concurrency; i++)
-       new_prefetch_pages += (double) io_concurrency / (double) i;
-
-   *target = new_prefetch_pages;
-
-   /* This range check shouldn't fail, but let's be paranoid */
-   return (new_prefetch_pages >= 0.0 && new_prefetch_pages < (double) INT_MAX);
-}
-
 /*
  * PrefetchBuffer -- initiate asynchronous read of a block of a relation
  *

diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 4c6d64866230dec5b7ea0221ae3b3b5af4423f78..326e773b25fc6361e91d7cbcf8ad71be3dd574a9 100644 (file)
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -196,7 +196,6 @@ static bool check_autovacuum_max_workers(int *newval, void **extra, GucSource so
 static bool check_max_wal_senders(int *newval, void **extra, GucSource source);
 static bool check_autovacuum_work_mem(int *newval, void **extra, GucSource source);
 static bool check_effective_io_concurrency(int *newval, void **extra, GucSource source);
-static void assign_effective_io_concurrency(int newval, void *extra);
 static void assign_pgstat_temp_directory(const char *newval, void *extra);
 static bool check_application_name(char **newval, void **extra, GucSource source);
 static void assign_application_name(const char *newval, void *extra);
@@ -2882,7 +2881,7 @@ static struct config_int ConfigureNamesInt[] =
        0,
 #endif
        0, MAX_IO_CONCURRENCY,
-       check_effective_io_concurrency, assign_effective_io_concurrency, NULL
+       check_effective_io_concurrency, NULL, NULL
    },
 
    {
@@ -11457,36 +11456,14 @@ check_max_worker_processes(int *newval, void **extra, GucSource source)
 static bool
 check_effective_io_concurrency(int *newval, void **extra, GucSource source)
 {
-#ifdef USE_PREFETCH
-   double      new_prefetch_pages;
-
-   if (ComputeIoConcurrency(*newval, &new_prefetch_pages))
-   {
-       int        *myextra = (int *) guc_malloc(ERROR, sizeof(int));
-
-       *myextra = (int) rint(new_prefetch_pages);
-       *extra = (void *) myextra;
-
-       return true;
-   }
-   else
-       return false;
-#else
+#ifndef USE_PREFETCH
    if (*newval != 0)
    {
        GUC_check_errdetail("effective_io_concurrency must be set to 0 on platforms that lack posix_fadvise().");
        return false;
    }
-   return true;
-#endif                         /* USE_PREFETCH */
-}
-
-static void
-assign_effective_io_concurrency(int newval, void *extra)
-{
-#ifdef USE_PREFETCH
-   target_prefetch_pages = *((int *) extra);
 #endif                         /* USE_PREFETCH */
+   return true;
 }
 
 static void

diff --git a/src/include/storage/bufmgr.h b/src/include/storage/bufmgr.h
index 73c7e9ba386b7a8f0cf29124da54c191df95b8cd..2bf5afdade6229987e840bbed6f33834c08702c5 100644 (file)
--- a/src/include/storage/bufmgr.h
+++ b/src/include/storage/bufmgr.h
@@ -57,7 +57,7 @@ extern bool zero_damaged_pages;
 extern int bgwriter_lru_maxpages;
 extern double bgwriter_lru_multiplier;
 extern bool track_io_timing;
-extern int target_prefetch_pages;
+extern int effective_io_concurrency;
 
 extern int checkpoint_flush_after;
 extern int backend_flush_after;
@@ -66,9 +66,6 @@ extern int    bgwriter_flush_after;
 /* in buf_init.c */
 extern PGDLLIMPORT char *BufferBlocks;
 
-/* in guc.c */
-extern int effective_io_concurrency;
-
 /* in localbuf.c */
 extern PGDLLIMPORT int NLocBuffer;
 extern PGDLLIMPORT Block *LocalBufferBlockPointers;
@@ -161,7 +158,6 @@ extern PGDLLIMPORT int32 *LocalRefCount;
 /*
  * prototypes for functions in bufmgr.c
  */
-extern bool ComputeIoConcurrency(int io_concurrency, double *target);
 extern void PrefetchBuffer(Relation reln, ForkNumber forkNum,
                           BlockNumber blockNum);
 extern Buffer ReadBuffer(Relation reln, BlockNumber blockNum);