what are pagecache, dentries, inodes?

Question

Just learned these 3 new techniques from https://unix.stackexchange.com/questions/87908/how-do-you-empty-the-buffers-and-cache-on-a-linux-system:

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To free pagecache

Answer 1

Hate to bring an old thread back from the dead, but I've been dealing with memory issues lately on my Linux Virtual Machines. Unfortunately, even with the virtualization of computing machines being great and the advancements of Linux memory and resource allocation being superb, conflicts occur when the hypervisor acts out what it calls "performance features".

VMWare will actively send RAM that hasn't been "written or modified" recently, to the disk. When your disk is on a SAN, that means reading from the RAM is now at 1Gbps to 10Gbps at best if you have a REALLY performant RAID and steady network access (ignoring the fact that now the RAM of say 100 VMs are all using the same SAN). DDR3 RAM operates at 25Gbps+ on modern systems, so I'll assume you can see the problem with systems running at 1/25th to less than 1/2 of the speed anticipated.

The caches on my linux systems are literally the same speed as disk I/O of the filesystem, meaning they do not help our performance and are actively sending the OS's RAM into Swap instead of clearing caches. This is a huge problem thanks to VMWare, not because of Linux, but be aware that cloud infrastructure often does stupid crap like this all the time unfortunately. You can read more here: https://www.vmware.com/content/dam/digitalmarketing/vmware/en/pdf/techpaper/perf-vsphere-memory_management.pdf or if you use VMWare, surely you'll notice the "allocated memory" vs "active memory" and where your VMs will always display a different amount than VMWare because of this distinction and treatment of the memory.