Update the documentation and implementation of swap calculation.

Make the ramps consistent between suspend-to-disk and not,
and don't do the weird drop from 8GiB swap down to 4GiB for
large-memory systems.

src/modules/partition/core/PartitionActions.cpp

@@ -45,17 +45,7 @@ using CalamaresUtils::operator""_MiB;
 qint64
 swapSuggestion( const qint64 availableSpaceB )
 {
-    /* If suspend-to-disk is demanded, then we always need enough
-     * swap to write the whole memory to disk -- between 2GB and 8GB
-     * RAM give proportionally more swap, and from 8GB RAM keep
-     * swap = RAM.
-     *
-     * If suspend-to-disk is not demanded, then ramp up more slowly,
-     * to 8GB swap at 16GB memory, and then drop to 4GB for "large
-     * memory" machines, on the assumption that those don't need swap
-     * because they have tons of memory (or whatever they are doing,
-     * had better not run into swap).
-     */
+    // See partition.conf for explanation
     qint64 suggestedSwapSizeB = 0;
     auto memory = CalamaresUtils::System::instance()->getTotalMemoryB();
     qint64 availableRamB = memory.first;
@@ -65,36 +55,25 @@ swapSuggestion( const qint64 availableSpaceB )
         Calamares::JobQueue::instance()->globalStorage()->
             value( "ensureSuspendToDisk" ).toBool();
 
-    if ( ensureSuspendToDisk )
-    {
-        if ( availableRamB < 4_GiB )
-            suggestedSwapSizeB = qMax( 2_GiB, availableRamB * 2 );
-        else if ( availableRamB >= 4_GiB && availableRamB < 8_GiB )
-            suggestedSwapSizeB = 8_GiB;
-        else
-            suggestedSwapSizeB = availableRamB;
+    // Ramp up quickly to 8GiB, then follow memory size
+    if ( availableRamB <= 4_GiB )
+        suggestedSwapSizeB = availableRamB * 2;
+    else if ( availableRamB <= 8_GiB )
+        suggestedSwapSizeB = 8_GiB;
+    else
+        suggestedSwapSizeB = availableRamB;
 
-        suggestedSwapSizeB *= overestimationFactor;
-    }
-    else //if we don't care about suspend to disk
-    {
-        if ( availableRamB < 2_GiB )
-            suggestedSwapSizeB = qMax( 2_GiB, availableRamB * 2 );
-        else if ( availableRamB >= 2_GiB && availableRamB < 8_GiB )
-            suggestedSwapSizeB = availableRamB;
-        else if ( availableRamB >= 8_GiB && availableRamB < 16_GiB )
-            suggestedSwapSizeB = 8_GiB;
-        else
-            suggestedSwapSizeB = 4_GiB;
+    // .. top out at 8GiB if we don't care about suspend
+    if ( !ensureSuspendToDisk )
+        suggestedSwapSizeB = qMin( 8_GiB, suggestedSwapSizeB );
 
-        suggestedSwapSizeB *= overestimationFactor;
 
-        // don't use more than 10% of available space
-        qreal maxSwapDiskRatio = 0.10;
-        qint64 maxSwapSizeB = availableSpaceB * maxSwapDiskRatio;
-        if ( suggestedSwapSizeB > maxSwapSizeB )
-            suggestedSwapSizeB = maxSwapSizeB;
-    }
+    // Allow for a fudge factor
+    suggestedSwapSizeB *= overestimationFactor;
+
+    // don't use more than 10% of available space
+    if ( !ensureSuspendToDisk )
+        suggestedSwapSizeB = qMin( suggestedSwapSizeB, qint64( 0.10 * availableSpaceB ) );
 
     cDebug() << "Suggested swap size:" << suggestedSwapSizeB / 1024. / 1024. / 1024. << "GiB";
 

src/modules/partition/partition.conf

@@ -6,11 +6,17 @@ efiSystemPartition:     "/boot/efi"
 # Make sure an autogenerated swap partition is big enough for hibernation in
 # automated partitioning modes. Swap can be disabled through *neverCreateSwap*.
 #
-# When *ensureSuspendToDisk* is true, swap is never smaller than physical
-# memory, follows the guideline 2 * memory until swap reaches 8GiB.
-# When *ensureSuspendToDisk* is false, swap size scales up with memory
-# size until 8GiB, then at roughly half of memory size.
-#
+#  - *neverCreateSwap* is true: no swap is created
+#  - *neverCreateSwap* is false (the default): swap is created, as follows:
+#    - *ensureSuspendToDisk* is true (default): Swap is always at least total
+#      memory size, and up to 4GiB RAM follows the rule-of-thumb 2 * memory;
+#      from 4GiB to 8 GiB it stays steady at 8GiB, and over 8 GiB memory
+#      swap is the size of main memory.
+#    - *ensureSuspendToDisk* is false: Follows the rules above, but Swap is at
+#      most 8GiB, and no more than 10% of available disk.
+#    In both cases, a fudge factor (usually 10% extra) is applied so that there
+#    is some space for administrative overhead (e.g. 8 GiB swap will allocate
+#    8.8GiB on disk in the end).
 #
 # Default is true.
 ensureSuspendToDisk:    true