读完之后，我决定尝试为Cassandra启用hugepages。然而，在设置了一些东西之后，Cassandra根本就没有开始。我怀疑我错过了一些重要的设置，使Cassandra实际上可以使用hugepages，但我不知道是什么。
具体来说，我做了：

echo 112 >  /proc/sys/vm/hugetlb_shm_group
echo 2048 > /proc/sys/vm/nr_hugepages
mount -t hugetlbfs hugetlbfs /dev/hugepages

其中112是cassandra组id，并将以下内容添加到cassandra-env.sh中：

JVM_OPTS="$JVM_OPTS -XX:+UseLargePages -XX:LargePageSizeInBytes=2m -XX:+AlwaysPreTouch"

我在debian stretch上使用cassandra 3.11.3。
我在看以下文件：
https://tobert.github.io/tldr/cassandra-java-huge-pages.html
https://docs.openstack.org/nova/rocky/admin/huge-pages.html
https://blog.pythian.com/performance-tuning-hugepages-in-linux/
https://gist.github.com/tobert/c13803b905069b82e1fd
注意：虽然/var/log/cassandra中没有任何内容，但我在/var/lib/cassandra/hs\u err\u 1557985546.log中发现了以下内容：

# There is insufficient memory for the Java Runtime Environment to continue.

# Native memory allocation (mmap) failed to map 3992977408 bytes for committing reserved memory.

# Possible reasons:

# The system is out of physical RAM or swap space

# In 32 bit mode, the process size limit was hit

# Possible solutions:

# Reduce memory load on the system

# Increase physical memory or swap space

# Check if swap backing store is full

# Use 64 bit Java on a 64 bit OS

# Decrease Java heap size (-Xmx/-Xms)

# Decrease number of Java threads

# Decrease Java thread stack sizes (-Xss)

# Set larger code cache with -XX:ReservedCodeCacheSize=

# This output file may be truncated or incomplete.

# 

# Out of Memory Error (os_linux.cpp:2643), pid=7022, tid=0x00007f10e1e02700

# 

# JRE version:  (8.0_181-b13) (build )

# Java VM: OpenJDK 64-Bit Server VM (25.181-b13 mixed mode linux-amd64 compressed oops)

.... some stack trace with just numbers ...
/proc/meminfo:
MemTotal:        7657756 kB
MemFree:         2251340 kB
MemAvailable:    2198368 kB
Buffers:           33656 kB
Cached:           102892 kB
SwapCached:            0 kB
Active:          1066784 kB
Inactive:          52664 kB
Active(anon):     983092 kB
Inactive(anon):      316 kB
Active(file):      83692 kB
Inactive(file):    52348 kB
Unevictable:           0 kB
Mlocked:               0 kB
SwapTotal:             0 kB
SwapFree:              0 kB
Dirty:             19108 kB
Writeback:             0 kB
AnonPages:        982900 kB
Mapped:            26556 kB
Shmem:               440 kB
Slab:              40420 kB
SReclaimable:      21728 kB
SUnreclaim:        18692 kB
KernelStack:        2976 kB
PageTables:         5312 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:     1731724 kB
Committed_AS:    1227816 kB
VmallocTotal:   34359738367 kB
VmallocUsed:           0 kB
VmallocChunk:          0 kB
HardwareCorrupted:     0 kB
AnonHugePages:         0 kB
ShmemHugePages:        0 kB
ShmemPmdMapped:        0 kB
HugePages_Total:    2048
HugePages_Free:     2047
HugePages_Rsvd:      119
HugePages_Surp:        0
Hugepagesize:       2048 kB
DirectMap4k:       71680 kB
DirectMap2M:     7919616 kB

所以问题似乎是它没有使用hugepages，并且没有足够的非巨大内存。但为什么呢？