Memory, Swap, Process, Thread, File, Data Storage and Tunning

Physical and virtual memory

Traditionally, one has physical memory, that is, memory that is actually present in the machine, and virtual memory, that is, address space. Usually the virtual memory is much larger than the physical memory, and some hardware or software mechanism makes sure that a program can transparently use this much larger virtual space while in fact only the physical memory is available.

Nowadays things are reversed: on a Pentium II one can have 64 GB physical memory, while addresses have 32 bits, so that the virtual memory has a size of 4 GB. We'll have to wait for a 64-bit architecture to get large amounts of virtual memory again. The present situation on a Pentium with more than 4 GB is that using the PAE (Physical Address Extension) it is possible to place the addressable 4 GB anywhere in the available memory, but it is impossible to have access to more than 4 GB at once.


Swap Space


Linux divides its physical RAM (random access memory) into chucks of memory called pages. Swapping is the process whereby a page of memory is copied to the preconfigured space on the hard disk, called swap space, to free up that page of memory. The combined sizes of the physical memory and the swap space is the amount of virtual memory available.


Linux has two forms of swap space: the swap partition and the swap file. The swap partition is an independent section of the hard disk used solely for swapping; no other files can reside there. The swap file is a special file in the filesystem that resides amongst your system and data files

How big should my swap space be?

Linux and other Unix-like operating systems use the term "swap" to describe both the act of moving memory pages between RAM and disk, and the region of a disk the pages are stored on. It is common to use a whole partition of a hard disk for swapping. However, with the 2.6 Linux kernel, swap files are just as fast as swap partitions. Now, many admins (both Windows and Linux/UNIX) follow an old rule of thumb that your swap partition should be twice the size of your main system RAM. Let us say I've 32GB RAM, should I set swap space to 64 GB? Is 64 GB of swap space really required? How big should your Linux / UNIX swap space be?

RAID 0, RAID 1, RAID 5, RAID 10

RAID 5 vs RAID 10: Recommended RAID For Safety and Performance

System Tuning Info for Linux Servers

Disk Tuning
File system Tuning
SCSI Tuning
Disk I/O Elevators
Network Interface Tuning
TCP Tuning
File limits
Process limits
Threads
NFS
Apache and other web servers
Samba
Openldap tuning
Sys V shm
Ptys and ttys
Benchmarks
System Monitoring

limit a Linux process run on a particular core

taskset  -p 

i.e.

taskset 1 -p 12345

to set process 12345 to use only processor/core 1

The bitmask can be a list (i.e. 1,3,4 to use cores 1 3 and 4 of a 4+ core system) or a bitmask in hex (0x0000000D the 1,3,4, 0x00000001 for just core 1)

taskset is usually in a package called shedutils.

Edit: almost forgot... If you want to set the affinity of a new command instead of change it for an existing process, use:

taskset   []...[]

Maximum number of threads per process in Linux

number of threads = total virtual memory / (stack size*1024*1024)

Java Process & Thread Limit on Linux

Maximum threads managed by java

Heap size                                                                                            

Java Heap size does not determine the amount of memory your process uses

If you monitor your java process with an OS tool like top or taskmanager, you may see the amount of memory you use exceed the amount you have specified for -Xmx. -Xmx limits the java heap size, java will allocate memory for other things, including a stack for each thread. It is not unusual for the total memory consumption of the VM to exceed the value of -Xmx.

Garbage collection

There are essentially two GC threads running. One is a very lightweight thread which does "little" collections primarily on the Eden (a.k.a. Young) generation of the heap. The other is the Full GC thread which traverses the entire heap when there is not enough memory left to allocate space for objects which get promoted from the Eden to the older generation(s).

If there is a memory leak or inadequate heap allocated, eventually the older generation will start to run out of room causing the Full GC thread to run (nearly) continuously.

The amount allocated for the Eden generation is the value specified with -Xmn. The amount allocated for the older generation is the value of -Xmx minus the -Xmn. Generally, you don't want the Eden to be too big or it will take too long for the GC to look through it for space that can be reclaimed.