Tune memory pressure settings for rapid mem usage changes

When playing an asset with video, frequent seek/jump operations within
a short period may cause rapid memory increases. The current pressure
settings may not allow (enough) memory release in time to avoid an
app running in a container to be killed due it reaching the memory
limits. To allow sufficient, in time, memory release, the release
needs to happen with the "synchrounous" flag set to true, allowing
full garbage collection cycle when reaching critical limits.

Furthermore, the critical threshold needs to be lowered as well
considering that the release is not instantaneous and on embedded
devices the 95% original threshold does not allow enough room for
mem release on apps with lower allowed memory usage limits.

Author: filipe-norte-red

Source/WTF/wtf/unix/MemoryPressureHandlerUnix.cpp

@@ -53,8 +53,13 @@ namespace WTF {
 // we wait longer to try again (s_maximumHoldOffTime).
 // These value seems reasonable and testing verifies that it throttles frequent
 // low memory events, greatly reducing CPU usage.
+#if PLATFORM(WPE)
+static const Seconds s_minimumHoldOffTime { 1_s };
+static const Seconds s_maximumHoldOffTime { 1_s };
+#else
 static const Seconds s_minimumHoldOffTime { 5_s };
 static const Seconds s_maximumHoldOffTime { 30_s };
+#endif
 static const size_t s_minimumBytesFreedToUseMinimumHoldOffTime = 1 * MB;
 static const unsigned s_holdOffMultiplier = 20;
 
@@ -69,7 +74,9 @@ void MemoryPressureHandler::triggerMemoryPressureEvent(bool isCritical)
     setMemoryPressureStatus(MemoryPressureStatus::SystemCritical);
 
     ensureOnMainThread([this, isCritical] {
-        respondToMemoryPressure(isCritical ? Critical::Yes : Critical::No);
+        // When memory usage reaches the critical state, we may not release enough memory in time if we use the
+        // async mode, so use synchrounous mode in such case
+        respondToMemoryPressure(isCritical ? Critical::Yes : Critical::No, isCritical ? Synchronous::Yes : Synchronous::No);
     });
 
     if (ReliefLogger::loggingEnabled() && isUnderMemoryPressure())

Source/WebKit/UIProcess/linux/MemoryPressureMonitor.cpp

@@ -48,8 +48,13 @@ static const Seconds s_minPollingInterval { 1_s };
 static const Seconds s_maxPollingInterval { 5_s };
 static const double s_minUsedMemoryPercentageForPolling = 50;
 static const double s_maxUsedMemoryPercentageForPolling = 85;
+#if PLATFORM(WPE)
+static const int s_memoryPresurePercentageThreshold = 80;
+static const int s_memoryPresurePercentageThresholdCritical = 85;
+#else
 static const int s_memoryPresurePercentageThreshold = 90;
 static const int s_memoryPresurePercentageThresholdCritical = 95;
+#endif
 // cgroups.7: The usual place for such mounts is under a tmpfs(5)
 // filesystem mounted at /sys/fs/cgroup.
 static const char* s_cgroupMemoryPath = "/sys/fs/cgroup/%s/%s/%s";