ns節省記憶體的小技巧

.

在ns user manual 24.4.2中介紹了一些節省記憶體的小技巧：

1. 避免trace-all：$ns trace-all $f會使得trace objects被塞到所有links，若你只想trace一個link，這個overhead是可以避免的，每個link約可節省14KB
2. 在使用系列的var時，盡量用array：ns中的每個var都有overhead，如set n$i [$ns node]中的n$i（就像n1, n2, n3, ...)，若需要系列的變數時，使用array取代，如n($si)（就像n(1), n(2), n(3), ...)，那麼其實只有一個變數會被使用(即n)，每個變數可以節省40+Btye
3. 避免宣告不需要的變數：若一個物件在後面不會再用到，那麼避免為該物件取名，每個變數可省約80B，如下列會為CtrMcast的物件指定一個變數：cmcast(1)，但這其實是不必要的set cmcast(1) [new CtrMcast $ns $n(1) $ctrmcastcomp [list 1 1]]
   可以代換成：
   new CtrMcast $ns $n(1) $ctrmcastcomp [list 1 1]
4. 在FreeBSD上run：在FreeBSD上malloc的overhead比其他的OS小，ns2的作者們最後還是會將FreeBSD的allocator port到其他平台
5. Dynamic binding： 在C++中使用bind()會使你創建的每個物件都消耗記憶體，若你創建許多相同的物件時，使用bind的代價太高。將bind()代換成delay_bind()，使得所需要的記憶體降到per-class level而非每個物件(我個人的解讀時，bind會使得所有物件都用到它，不管需不需要，就像global variable的宣告，而delay_bind則是當宣告的物件有用到它時，也就是宣告的物件為該class時，這時才真的宣告=>這時才秏記憶體)，從ns/object.cc參考如何binding的範例
6. disabling packet header：在大量packet的simulation中，將你simulation中不會用到的packet header取消將會大量的節省記憶體(ns default是將所有packet header打開
   可在宣告ns前(set ns [new Simulator])輸入：
   remove-all-packet-headers，將所有packet header關閉(在ns宣告前輸入才有用)
   再將需要的packet header加回來，如IP、TCP：
   add-packet-header IP TCP Flags

原文如下：

Some tips to saving memory (some of these use examples from the cmcast-100.tcl script): If you have many links or nodes:

1. Avoid trace-all : $ns trace-all $f causes trace objects to be pushed on all links. If you only want to trace one link, there’s no need for this overhead. Saving is about 14 KB/link.
2. Use arrays for sequences of variables : Each variable, say n$i in set n$i [$ns node], has a certain overhead. If a sequence of nodes are created as an array, i.e. n($i), then only one variable is created, consuming much less memory. Saving is about 40+ Byte/variable.
3. Avoid unnecessary variables : If an object will not be referred to later on, avoid naming the object. E.g. set cmcast(1) [new CtrMcast $ns $n(1) $ctrmcastcomp [list 1 1]] would be better if replaced by new CtrMcast $ns $n(1) $ctrmcastcomp [list 1 1]. Saving is about 80 Byte/variable.
4. Run on top of FreeBSD : malloc() overhead on FreeBSD is less than on some other systems. We will eventually port that allocator to other platofrms.
5. Dynamic binding : Using bind() in C++ consumes memory for each object you create. This approach can be very expensive if you create many identical objects. Changing bind() to delay_bind() changes this memory requirement to per-class. See ns/object.cc for an example of how to do binding, either way.
6. Disabling packet headers : For packet-intensive simulations, disabling all packet headers that you will not use in your simulation may significantly reduce memory usage. See Section 12.1 for detail.

.