Use RAMdisk based on available memory instead of free memory (#1463)

Resolves #1461

Based on `man free` it is more appropriate to use the metric for
available memory instead of free memory, when deciding whether to use
the RAMdisk for TinyPilot installation/update.

>   free   Unused memory (MemFree and SwapFree in /proc/meminfo)
> ...
> available: Estimation of how much memory is available for starting new
applications, without swapping. Unlike the data provided by the cache or
free fields, this field takes into account page cache and also that not
all reclaimable memory slabs will be reclaimed due to items being in use
(MemAvailable in /proc/meminfo, available on kernels 3.14, emulated on
kernels 2.6.27+, otherwise the same as free)

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get-tinypilot.sh

@@ -71,11 +71,11 @@ readonly LEGACY_INSTALLER_DIR='/opt/tinypilot-updater'
 #   du --summarize --total --bytes "${INSTALLER_DIR}" "${BUNDLE_FILE}"
 readonly RAMDISK_SIZE_MIB=500
 
-FREE_MEMORY_MIB="$(free --mebi |
+AVAILABLE_MEMORY_MIB="$(free --mebi |
   grep --fixed-strings 'Mem:' |
   tr --squeeze-repeats ' ' |
-  cut --delimiter ' ' --fields 4)"
-readonly FREE_MEMORY_MIB
+  cut --delimiter ' ' --fields 7)"
+readonly AVAILABLE_MEMORY_MIB
 
 # Assign a provisional installation directory for our `clean_up` function.
 INSTALLER_DIR='/mnt/tinypilot-installer'
@@ -93,7 +93,7 @@ trap 'clean_up' EXIT
 
 # Determine the installation directory. Use RAMdisk if there is enough memory,
 # otherwise, fall back to regular disk.
-if (( "${FREE_MEMORY_MIB}" >= "${RAMDISK_SIZE_MIB}" )); then
+if (( "${AVAILABLE_MEMORY_MIB}" >= "${RAMDISK_SIZE_MIB}" )); then
   # Mount volatile RAMdisk.
   # Note: `tmpfs` can use swap space when the device's physical memory is under
   # pressure. Alternatively, we could use `ramfs` which doesn't use swap space,

scripts/install-bundle

@@ -86,11 +86,11 @@ readonly LEGACY_INSTALLER_DIR='/opt/tinypilot-updater'
 #   du --summarize --total --bytes "${INSTALLER_DIR}" "${BUNDLE_FILE}"
 readonly RAMDISK_SIZE_MIB=500
 
-FREE_MEMORY_MIB="$(free --mebi |
+AVAILABLE_MEMORY_MIB="$(free --mebi |
   grep --fixed-strings 'Mem:' |
   tr --squeeze-repeats ' ' |
-  cut --delimiter ' ' --fields 4)"
-readonly FREE_MEMORY_MIB
+  cut --delimiter ' ' --fields 7)"
+readonly AVAILABLE_MEMORY_MIB
 
 # Assign a provisional installation directory for our `clean_up` function.
 INSTALLER_DIR='/mnt/tinypilot-installer'
@@ -108,7 +108,7 @@ trap 'clean_up' EXIT
 
 # Determine the installation directory. Use RAMdisk if there is enough memory,
 # otherwise, fall back to regular disk.
-if (( "${FREE_MEMORY_MIB}" >= "${RAMDISK_SIZE_MIB}" )); then
+if (( "${AVAILABLE_MEMORY_MIB}" >= "${RAMDISK_SIZE_MIB}" )); then
   # Mount volatile RAMdisk.
   # Note: `tmpfs` can use swap space when the device's physical memory is under
   # pressure. Alternatively, we could use `ramfs` which doesn't use swap space,