From fe39a007d862ccea80789ff6e1e50d065b4a59ab Mon Sep 17 00:00:00 2001
From: Dirk Zimoch <dirk.zimoch@psi.ch>
Date: Mon, 9 Feb 2026 14:27:22 +0100
Subject: [PATCH] Allow percentages for callbackParallelThreads count

---
 documentation/new-notes/PR-788.md           | 13 ++++++++++
 modules/database/src/ioc/db/callback.h      |  5 +++-
 modules/database/src/ioc/db/dbIocRegister.c | 27 +++++++++++++++++++--
 3 files changed, 42 insertions(+), 3 deletions(-)

diff --git a/documentation/new-notes/PR-788.md b/documentation/new-notes/PR-788.md
index 9dc41588c..d5b83c296 100644
--- a/documentation/new-notes/PR-788.md
+++ b/documentation/new-notes/PR-788.md
@@ -6,3 +6,16 @@ That fixes the problem that an IOC which was started with a limited affinity
 mask (e.g. using `taskset`) and that called `callbackParallelThreads` with an
 argument of 0 in the startup script, had started more parallel callback
 threads than it has CPU cores available.
+
+The iocsh function `callbackParallelThreads` now also accepts positive or
+negative percentages for `count`, referring to the availabple number of cores
+and always rounding the number of used cores down with a minimum of 1.
+
+#### Examples
+
+```text
+# use up to 25% of available cpu cores for callback threads
+callbackParallelThreads 25%
+# keep at least 10% of available cpu cores unused for callback threads
+callbackParallelThreads -10%
+```
diff --git a/modules/database/src/ioc/db/callback.h b/modules/database/src/ioc/db/callback.h
index 53595c5a9..54f6a8a32 100644
--- a/modules/database/src/ioc/db/callback.h
+++ b/modules/database/src/ioc/db/callback.h
@@ -199,7 +199,10 @@ DBCORE_API void callbackQueueShow(const int reset);
  * Calling with count<0 computes the count based on the number of CPU cores
  * currently available to the calling thread,
  * eg. Passing -2 on an 8 core system will start 6 worker threads.
- * In all cases, at least one worker thread will always run.
+ * The iocsh wrapper also accepts positive or negative percentages for count,
+ * which allows to refer to a fraction of the currently available CPU cores,
+ * always rounding the number of created threads down.
+ * In any case, at least one worker thread will always run.
  *
  * An empty prio name or the special value "*" will modify all priorities.
  * Otherwise, only the named priority is modified.
diff --git a/modules/database/src/ioc/db/dbIocRegister.c b/modules/database/src/ioc/db/dbIocRegister.c
index abdc28fa7..a6194cbac 100644
--- a/modules/database/src/ioc/db/dbIocRegister.c
+++ b/modules/database/src/ioc/db/dbIocRegister.c
@@ -10,6 +10,8 @@
 
 #define EPICS_PRIVATE_API
 
+#include <string.h>
+
 #include "iocsh.h"
 
 #include "callback.h"
@@ -27,6 +29,9 @@
 #include "dbState.h"
 #include "db_test.h"
 #include "dbTest.h"
+#include "epicsStdlib.h"
+#include "epicsStdio.h"
+#include "errlog.h"
 
 DBCORE_API extern int callbackParallelThreadsDefault;
 
@@ -503,7 +508,7 @@ static void callbackQueueShowCallFunc(const iocshArgBuf *args)
 }
 
 /* callbackParallelThreads */
-static const iocshArg callbackParallelThreadsArg0 = { "no of threads", iocshArgInt};
+static const iocshArg callbackParallelThreadsArg0 = { "no of threads", iocshArgString};
 static const iocshArg callbackParallelThreadsArg1 = { "priority", iocshArgString};
 static const iocshArg * const callbackParallelThreadsArgs[2] =
     {&callbackParallelThreadsArg0,&callbackParallelThreadsArg1};
@@ -513,7 +518,25 @@ static const iocshFuncDef callbackParallelThreadsFuncDef = {"callbackParallelThr
                                                             "or one of LOW, MEDIUM, or HIGH.\n"};
 static void callbackParallelThreadsCallFunc(const iocshArgBuf *args)
 {
-    iocshSetError(callbackParallelThreads(args[0].ival, args[1].sval));
+    epicsInt32 num = 0;
+    if (args[0].sval) {
+        char* end;
+        long status = epicsParseInt32(args[0].sval, &num, 10, &end);
+        if (status || (end[0] && strcmp(end, "%") != 0))
+        {
+            fprintf(epicsGetStderr(),
+                ANSI_RED("Invalid integer '%s'.\n"), args[0].sval);
+            iocshSetError(-1);
+            return;
+        }
+        if (end[0] == '%') {
+            /* round down, min 1 */
+            if (num < 0) num += 100;
+            if (num > 0) num = num * epicsThreadGetCPUs() / 100;
+            if (num <= 0) num = 1;
+        }
+    }
+    iocshSetError(callbackParallelThreads(num, args[1].sval));
 }
 
 /* dbStateCreate */