HOW TO OPTIMIZE CloudLinux OS Limits

• What is LVE

• mod_hostinglimits is Apache

module that:o detects VH

o puts apache process into LVE

o lets apache server it

o removes apache process from LVE

UNDERSTANDING LVE

• LVE limits don’t depend on hardware

• Default LVE limits optimized for regular shared

hosting account

o More powerful hardware → more clients per server

o Less powerful hardware → less clients

• Low end customers → smaller limits

• High end customers → larger limits

TYPICAL Hosting AccountSPEED=100%PMEM=256MBVMEM=0IO=1024KB/sNPROC=100EP=20

HIGH END Hosting AccountSPEED=200%PMEM=512MBVMEM=0IO=1024KB/sNPROC=100EP=40

Limits <-> Hardware dependency

• PHP/CGI scripts

• SSH sessions

• Cron jobs

What do limits “LIMIT”

• E-Maile Pipes (cPanel only)

• MySQL queries (requires MySQL

Governor, CPU & IO only)

Static content is NOT limited by defaulto Noticeable overheado Can be changed by setting AllowHandler * in modhostinglimits.conf

LIMITS UNITS DEFAULT VALUE

SPEED % of a core, or HZ 100%

PMEM KB 256MB

VMEM KB 0MB

IO KB/sec 1024KB/sec

IOPS [requires lve1.3+] Operations per second 1024

NPROC number 100

EP number 20

Default Limits

Type RLIMIT LVE Limit

CPU Per process. Amount of seconds each process will execute.

Whole account. Total amount of CPU all processes within account can use at the same time.

Memory Per process, limits memory consumption per process.

Total memory consumption for all processes within accounts.

IO No alternative Amount of data per second that can be read/written

IOPS No alternative Number of IO operations / second allowed

Number of processes

Max number of processes per account Max number of processes per account

Entry processes No alternatives Max number of apache connections for account

Difference between RLimit & LVE Limits

• % of a single core OR # in hzo speed=150% → 1 and ½ cores

o 2ghz → 2ghz, no matter what the speed of CPU is

• Default → 100% (1 core)• Recommended SPEED settings → 100% for regular

accounts, 200% for high performance accounts

• Once limit reached, processes slow down

• Hyper threading counts as a separate core

SPEED (CPU LIMIT)

HZ settings allows to set approximately same performance level across different hardware

• Single thread can occupy one core

o Using limit < 1 core will cause

automatic throttling of all requests

• ½ core → requests take 2x as slow

• Due to context switching → CPU

usage is higher than without the

limit

DANGER OF LOW LIMIT

• Limits should be at most ½ of all cores

• Better 1/4 of all cores

WHAT IF LIMIT TOO HIGH

If limit >= ½ cores –

you need just 2 accounts to OVERLOAD server

• Disc throughput○ Read & Write

○ KB/s

• Default: 1024KB/sec

• Processes throttled on IO

once limit is hit

IO LIMIT

• SSD provides better throughput○ HDD → 30-100MB/s

○ SSD → 80-500MB/s

○ Limits are still per account / might not have to

be changed

IO limits Standard account High-end account

HDD 1 MB/s 5 MB/s

SSD 4 MB/s 10 MB/s

SSD vs HDD Disks [IO]

• Disc throughputo Read & Write

• Restricts total number of IO

operations

o Operations stop once limit is reached, until

second expires

• Default: 1024 operations per second

IOPS LIMIT

SSD vs HDD Disks [IOPS]

• SSD provides significantly higher IOPS rate○ HDD → 100 to 400 IOPS

○ SDD → 5,000 to 100,000 IOPS Source: https://en.wikipedia.org/wiki/IOPS

https://calypsotesters.com/summary-performance-comparison-hdd-sshd-ssd/

○ Limits are still per account / might not have to be

changed Yet, you can…

vMEM vs pMEM

• vMEM -- virtual memory (deprecated on CL6 & CL7)

vMEM → allocated memory, often not used. Prevents process from

allocating more memory

• pMEM -- physical memory

pMEM → used memory. Counts actual memory of pages in use. Will

kill (OOM) processes for account, if pMEM limit is reached

• Default: 256MB

• RSS field in ps, RES in top.

○ Includes shared memory

○ Includes disk caches

Used for account, caches will be freed if user

reached memory limit

pMEM limit

• Number of processes limit

• Default: 100• Most users will never hit this limit

○ Safe to increases to 1000...

NPROC Limit

PROTECTS AGAINST FORK BOMBS AND SIMILAR ATTACKS

• Number of entry processes

(Apache connections)

• Default: 20• Error code 508 response when

limit is reached

• Protects against exhaustion of

Apache slots (MaxClients)

EP LIMIT

Questions?

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