How to use the Memory Allocation Simulator
- 1. Enter blocks and processesBlock sizes in memory order, and process sizes in arrival order.
- 2. Pick optionsOne algorithm or all four, and fixed or variable partitions.
- 3. CompareSee each placement, the leftover holes and who has to wait.
The four algorithms
- First fit: use the first block that is big enough. Fast and simple.
- Best fit: use the smallest block that is big enough, leaving the smallest hole.
- Worst fit: use the largest block, leaving the biggest hole, in the hope that it's still useful.
- Next fit: like first fit, but start searching where the last search stopped.
With variable partitions, a process takes only what it needs and the rest of the block stays free as a smaller hole. A process may still have to wait even when the total free memory is enough, because no single hole is big enough: that's external fragmentation. With fixed partitions, each block holds one process and the unused space inside it is wasted: internal fragmentation.
Worked example
The sample is the classic textbook question: blocks of 100, 500, 200, 300 and 600 KB, and processes of 212, 417, 112 and 426 KB, with variable partitions:
- First fit: 212 → 500 (288 left), 417 → 600, 112 → the 288 hole, and 426 must wait.
- Best fit: 212 → 300, 417 → 500, 112 → 200 and 426 → 600. All 4 are placed.
- Worst fit: 212 → 600 (388 left), 417 → 500, 112 → the 388 hole, and 426 must wait.
Here best fit is the only one that fits everything, which is the textbook's point: no single algorithm always wins. Switch to fixed partitions to see internal fragmentation instead; for example first fit then puts 112 in the 200 block and wastes 88 KB.
Frequently asked questions
Which algorithm is best in practice?
Studies usually find first fit and best fit similar in memory use, with first fit faster. Worst fit tends to do worse.
What is compaction?
Moving processes together so the free holes join into one big block. It fixes external fragmentation but costs time.
Is this how malloc works?
Real allocators use more advanced schemes, such as size classes and free lists, but they face the same trade-offs.
Is the memory allocation simulator free to use?
Yes, the memory allocation simulator is free and costs nothing. Your first simulation shows a short form for your name and WhatsApp number one time; once that's done, this tool and every other Uncodemy tool open with no form.