[fix](memory) Modified low water mark=min(6.4G, MemTotal * 5%) (apach…

…e#37525)

On 64G machine, warning water mark is 12.8G. when system available
memory reaches the warning water mark, load memtable will flush, this is
not a good idea.

`mem_limit` default 10%, except for the memory used by the Linux kernel
, buffer may be around 5%, so low water mark=min(6.4G, MemTotal * 5%)
looks better.

be/src/common/config.h

@@ -156,9 +156,9 @@ DECLARE_mInt32(max_fill_rate);
 DECLARE_mInt32(double_resize_threshold);
 
 // The maximum low water mark of the system `/proc/meminfo/MemAvailable`, Unit byte, default 6.4G,
-// actual low water mark=min(6.4G, MemTotal * 10%), avoid wasting too much memory on machines
-// with large memory larger than 64G.
-// Turn up max. On machines with more than 64G memory, more memory buffers will be reserved for Full GC.
+// actual low water mark=min(6.4G, MemTotal * 5%), avoid wasting too much memory on machines
+// with large memory larger than 128G.
+// Turn up max. On machines with more than 128G memory, more memory buffers will be reserved for Full GC.
 // Turn down max. will use as much memory as possible.
 DECLARE_Int64(max_sys_mem_available_low_water_mark_bytes);
 

be/src/util/mem_info.cpp

@@ -304,7 +304,7 @@ void MemInfo::init() {
         // upper sys_mem_available_low_water_mark, avoid wasting too much memory.
         _s_sys_mem_available_low_water_mark = std::max<int64_t>(
                 std::min<int64_t>(std::min<int64_t>(_s_physical_mem - _s_mem_limit,
-                                                    int64_t(_s_physical_mem * 0.1)),
+                                                    int64_t(_s_physical_mem * 0.05)),
                                   config::max_sys_mem_available_low_water_mark_bytes),
                 0);
         _s_sys_mem_available_warning_water_mark = _s_sys_mem_available_low_water_mark * 2;