3.cassandra遇到内存占用过高的问题

目前cssandra的内存分配如下：

https://docs.datastax.com/en/cassandra/2.1/cassandra/operations/ops_tune_jvm_c.html#opsTuneJVM__tuning-the-java-heap

# some systems like the raspberry pi don't report cores, use at least 1

if [ "$system_cpu_cores" -lt "1" ]

then

system_cpu_cores="1"

fi

# set max heap size based on the following

# max(min(1/2 ram, 1024MB), min(1/4 ram, 8GB))

# calculate 1/2 ram and cap to 1024MB

# calculate 1/4 ram and cap to 8192MB

# pick the max

half_system_memory_in_mb=`expr $system_memory_in_mb / 2`

quarter_system_memory_in_mb=`expr $half_system_memory_in_mb / 2`

if [ "$half_system_memory_in_mb" -gt "1024" ]

then

half_system_memory_in_mb="1024"

fi

if [ "$quarter_system_memory_in_mb" -gt "8192" ]

then

quarter_system_memory_in_mb="8192"

fi

if [ "$half_system_memory_in_mb" -gt "$quarter_system_memory_in_mb" ]

then

max_heap_size_in_mb="$half_system_memory_in_mb"

else

max_heap_size_in_mb="$quarter_system_memory_in_mb"

fi

#MAX_HEAP_SIZE="${max_heap_size_in_mb}M"

# Young gen: min(max_sensible_per_modern_cpu_core * num_cores, 1/4 * heap size)

max_sensible_yg_per_core_in_mb="100"

max_sensible_yg_in_mb=`expr $max_sensible_yg_per_core_in_mb "*" $system_cpu_cores`

desired_yg_in_mb=`expr $max_heap_size_in_mb / 4`

if [ "$desired_yg_in_mb" -gt "$max_sensible_yg_in_mb" ]

then

HEAP_NEWSIZE="${max_sensible_yg_in_mb}M"

else

HEAP_NEWSIZE="${desired_yg_in_mb}M"

fi

通过阅读代码可以知道，

MAX_HEAP_SIZE:

A = 如果1/2的内存都比1024M要小，则取1/2的内存，否则取1024

B = 如果1/4的内存比8g要小，则取1/4的内存，否则取8G

再取MAX（A,B）

可以看出当最小时取1/2的内存（1/2的内存比1024M小），最大的内存取8G

HEAP_NEWSIZE = 核数*100M

如果MAX_HEAP_SIZE/4<核数*100M,则取MAX_HEAP_SIZE/4

改为MAX_HEAP_SIZE = 512M就可以正常运行了，这样就不会出现内存不够的情况，但是官方标配是：

！！！！！！！内存不够就不要用分布式数据库