Re: [squid-users] Re: How much disk space is reclaimed by the cache replacement algorithms?
RW wrote: On Wed, 23 Dec 2009 13:44:54 -0900 Chris Robertson crobert...@gci.net wrote: RW wrote: On Wed, 23 Dec 2009 17:58:41 +0100 Matus UHLAR - fantomas uh...@fantomas.sk wrote: 1. How often is the replacement done? when the percentage of configured cache_dir size crosses the cache_swap_high value. I haven't looked at the code for a long-time and I only looked at 2.x, but cache_swap_high didn't seem to play any part that I could see. It's certainly not a simple high/low watermark algorithm. From http://www.squid-cache.org/Doc/config/cache_swap_high/... The low- and high-water marks for cache object replacement. Replacement begins when the swap (disk) usage is above the low-water mark and attempts to maintain utilization near the low-water mark. As swap utilization gets close to high-water mark object eviction becomes more aggressive. If utilization is close to the low-water mark less replacement is done each time. But as I said cache_swap_high doesn't actually do anything in the 2.x code, and given that that description has been around a long time I doubt anything has changed in 3.x. Garbage collection occurs at specific intervals. Default is hourly. AFAIUI (from the paper, not the code)... between high and low items are checked for freshness according to the replacement algorithm. Above high watermark everything marked for potential removal by the replacement algorithm is removed regardless of staleness. So ... * the low-volume cache stays low because live requests are performing the replacement of stale objects instead of garbage collection. * between low and high water marks an old but fresh object will be retained over a newer but definitely stale object. Amos -- Please be using Current Stable Squid 2.7.STABLE7 or 3.0.STABLE20 Current Beta Squid 3.1.0.15
Re: [squid-users] Re: How much disk space is reclaimed by the cache replacement algorithms?
RW wrote: On Wed, 23 Dec 2009 17:58:41 +0100 Matus UHLAR - fantomas uh...@fantomas.sk wrote: 1. How often is the replacement done? when the percentage of configured cache_dir size crosses the cache_swap_high value. I haven't looked at the code for a long-time and I only looked at 2.x, but cache_swap_high didn't seem to play any part that I could see. It's certainly not a simple high/low watermark algorithm. If you run a low-volume cache the percentage never budges from cache_swap_low (give or take a few objects), and if you shrink the cache size a little, it corrects within a couple of minutes, whether or not you hit the new cache_swap_high. From http://www.squid-cache.org/Doc/config/cache_swap_high/... The low- and high-water marks for cache object replacement. Replacement begins when the swap (disk) usage is above the low-water mark and attempts to maintain utilization near the low-water mark. As swap utilization gets close to high-water mark object eviction becomes more aggressive. If utilization is close to the low-water mark less replacement is done each time. Defaults are 90% and 95%. If you have a large cache, 5% could be hundreds of MB. If this is the case you may wish to set these numbers closer together. Hope that helps clear it up. Chris
