Linux: drop caches do not free buffers/cache - why?

Question

Since I've found discussion of buffers/cache on that site, not Unix of SE, I post my question here. I've read In Linux, what is the difference between "buffers" and "cache" reported by the free command? and Meaning of the buffers/cache line in the output of free, where it is written:

caches will be freed automatically if memory gets scarce, so they do not really matter.

Currently free reports 8Gb of buffers/cache, however, system when approaching zero free memory becomes unresponsive for long time and sync; echo 3 > /proc/sys/vm/drop_caches does not change much. Why? I post output of free -m and also more detailed output of cat /proc/meminfo:

              total        used        free      shared  buff/cache   available
Mem:          15740        4508        2366        8453        8865        2474

MemTotal:       16118172 kB
MemFree:          528472 kB
MemAvailable:     475820 kB
Buffers:            1588 kB
Cached:          8939100 kB
SwapCached:            0 kB
Active:          6711540 kB
Inactive:        8440460 kB
Active(anon):    6621624 kB
Inactive(anon):  8402256 kB
Active(file):      89916 kB
Inactive(file):    38204 kB
Unevictable:           0 kB
Mlocked:               0 kB
SwapTotal:             0 kB
SwapFree:              0 kB
Dirty:                 0 kB
Writeback:             0 kB
AnonPages:       6211412 kB
Mapped:          1534592 kB
Shmem:           8812568 kB
Slab:             203244 kB
SReclaimable:     106932 kB
SUnreclaim:        96312 kB
KernelStack:       18736 kB
PageTables:        93880 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:     8059084 kB
Committed_AS:   23933660 kB
VmallocTotal:   34359738367 kB
VmallocUsed:           0 kB
VmallocChunk:          0 kB
HardwareCorrupted:     0 kB
AnonHugePages:         0 kB
ShmemHugePages:        0 kB
ShmemPmdMapped:        0 kB
CmaTotal:              0 kB
CmaFree:               0 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB
DirectMap4k:     1761344 kB
DirectMap2M:    14704640 kB
DirectMap1G:     1048576 kB

Answer 1

/proc/sys/vm/drop_caches does not have a use in system operations. Don't use it, only makes things slower.

Well, it does have some uses, but they tend to be corner cases. Testing storage with a cold cache, unusual virtual memory workloads. See previously on Server Fault: Why drop caches in Linux?

---

Even ignoring Cached as if it were zero, this system already has relatively high memory utilization. AnonPages + Shmem is 14 GB and changes, alone only 1 GB less than MemTotal. And indeed, add in other stuff and MemAvailable is 0.5 GB.

One guess at how much RAM would be enough is from the kernel, Committed_AS. Enough to not page out. Which can be a problematic metric, but it is one of the few that directly estimates physical RAM usage. From your meminfo output, about 23 GB, or 150% of MemTotal. Half again what you have might be fine for some workloads, but I would be uncomfortable on a system with no swap and suspected memory capacity issues.

Some totals add up to more than 100% because the virtual memory system is being both lazy and clever. But only so much it can do. Shared memory (presumably a database) and apps anonymous memory adding to nearly as much as the host risks miserable performance.