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Table of Contents
PFSense - Squid - Options which affect the Cache
Cache Replacement Policy
Decides which objects will remain in cache and which objects are replaced to create space for the new objects.
- Default: Heap LFUDA.
The cache replacement policies are:
| Policy | Explanation | Comment |
|---|---|---|
| LRU | Squid's original list based LRU policy | Keeps recently referenced objects. |
| Heap GDSF | Greedy-Dual Size Frequency | Optimizes object hit rate by keeping smaller popular objects in cache so it has a better chance of getting a hit. It achieves a lower byte hit rate than LFUDA though since it evicts larger (possibly popular) objects. |
| Heap LFUDA | Least Frequently Used with Dynamic Aging | Keeps popular objects in cache regardless of their size and thus optimizes byte hit rate at the expense of hit rate since one large, popular object will prevent many smaller, slightly less popular objects from being cached. If using the LFUDA replacement policy you should increase the value of maximum_object_size above its default of 4096 KB to maximize the potential byte hit rate improvement of LFUDA. |
| Heap LRU | LRU policy implemented using a heap |
The policies utilize a dynamic aging mechanism that prevents cache pollution that can otherwise occur with frequency-based replacement policies.
See https://www.hpl.hp.com/techreports/1999/HPL-1999-69.html and https://fog.hpl.external.hp.com/techreports/98/HPL-98-173.html.
Low-Water Mark in %
The low-water mark for AUFS/UFS/diskd cache object eviction by the cache_replacement_policy algorithm.
- Default: 90.
High-Water Mark in %
The high-water mark for AUFS/UFS/diskd cache object eviction by the cache_replacement_policy algorithm.
- Default: 95.
Hard Disk Cache Size
Amount of disk space (in megabytes) to use for cached objects.
Set to 50% of the hard disk size.
Hard Disk Cache System (cache_dir)
This specifies the kind of storage system to use.
| Storage System | Description | Comment |
|---|---|---|
| usf | The old well-known Squid storage format that has always been there. | |
| aufs | Uses POSIX threads to avoid blocking the main Squid process on disk I/O. (Formerly known as async-io.) | |
| diskd | Uses a separate process to avoid blocking the main Squid process on disk I/O. | |
| null | Does not use any storage. Ideal for embedded. |
Level 1 Directories
Specifies the number of Level 1 directories for the hard disk cache.
- Default: 16.
Each level-1 directory contains 256 subdirectories, so a value of 256 level-1 directories will use a total of 65536 directories for the hard disk cache.
This will significantly slow down the startup process of the proxy service, but can speed up the caching under certain conditions.
Minimum Object Size
Objects smaller than the size specified (in kilobytes) will not be saved on disk.
- Default: 0 (meaning there is no minimum).
Maximum Object Size
Objects larger than the size specified (in megabytes) will not be saved on disk.
- Default: 4 (MB).
Memory Cache Size (cache_mem)
Specifies the ideal amount of physical RAM (in megabytes) to be used for In-Transit objects, Hot Objects and Negative-Cached objects.
- Default is : 64MB.
Data for these objects are stored in 4 KB blocks. This parameter specifies the ideal upper limit on the total size of 4 KB blocks allocated.
In-transit objects have priority over the others. When additional space is needed for incoming data, Negative-cached and Hot objects will be released. In other words, the negative-cached and hot objects will fill up any unused space not needed for In-transit objects.
If circumstances require, this limit will be exceeded. Specifically, if your incoming request rate requires more than cache_mem of memory to hold In-transit objects, Squid will exceed this limit to satisfy the new requests. When the load decreases, blocks will be freed until the high-water mark is reached. Thereafter, blocks will be used to store hot objects.
NOTE: This tag does not specify the maximum process size.
It places a limit on one aspect of squid's memory usage.
Squid uses memory for other things as well.
Process will probably become twice or three times bigger than the value put here.
Maximum Object Size in RAM
Objects greater than this size (in kilobytes) will not be attempted to kept in the memory cache.
- Default: 256 (KB).
Memory Replacement Policy
The memory replacement policy determines which objects are purged from memory when space is needed.
- Default: heap GDSF.
The cache replacement policies are same as cache_replacemen_policy:
| Policy | Explanation | Comment |
|---|---|---|
| LRU | Squid's original list based LRU policy | Keeps recently referenced objects. |
| Heap GDSF | Greedy-Dual Size Frequency | Optimizes object hit rate by keeping smaller popular objects in cache so it has a better chance of getting a hit. It achieves a lower byte hit rate than LFUDA though since it evicts larger (possibly popular) objects. |
| Heap LFUDA | Least Frequently Used with Dynamic Aging | Keeps popular objects in cache regardless of their size and thus optimizes byte hit rate at the expense of hit rate since one large, popular object will prevent many smaller, slightly less popular objects from being cached. If using the LFUDA replacement policy you should increase the value of maximum_object_size above its default of 4096 KB to maximize the potential byte hit rate improvement of LFUDA. |
| Heap LRU | LRU policy implemented using a heap |
The policies utilize a dynamic aging mechanism that prevents cache pollution that can otherwise occur with frequency-based replacement policies.
See https://www.hpl.hp.com/techreports/1999/HPL-1999-69.html and https://fog.hpl.external.hp.com/techreports/98/HPL-98-173.html.
Cache Dynamic Content
With dynamic cache enabled, you can also apply refresh_patterns to sites like Windows Updates.