Thanks for explaining! Looks like this is too complex to be reliably tricked by a few dd commands.
Now I'm resorting to "sysctl kern.bufcachepercent=5; sysctl kern.bufcachepercent=90" to "almost" clear the cache. If only setting it to 0 were allowed... On Tue, Jul 3, 2018 at 3:06 PM, Claudio Jeker <cje...@diehard.n-r-g.com> wrote: > On Tue, Jul 03, 2018 at 01:30:20PM +0200, Maximilian Pichler wrote: >> On Tue, Jul 3, 2018 at 11:47 AM, Janne Johansson <icepic...@gmail.com> wrote: >> > https://www.tedunangst.com/flak/post/2Q-buffer-cache-algorithm >> >> Thanks. If I'm reading this correctly upon access (read or write), an >> action is performed depending on what queue a buffer is in: >> - none: Take a buffer from the tail of the cold queue and insert it at >> the front of the hot queue. >> - hot: Keep it there, but move to the front. >> - cold: Move it to the front of the warm queue. >> - warm: Keep it there, but move to the front. >> >> If a buffer reaches the tail of a queue it moves on like this: >> hot -> cold >> cold -> (delete) >> warm -> cold >> >> Based on this understanding, let me describe my (failed) attempt to >> remove a file from cache. >> >> First, let's use 90%: >> $ doas sysctl kern.bufcachepercent >> kern.bufcachepercent=90 >> >> My machine has 16GB of RAM, but the end of the blog post says >> something about himem, so maybe this means only 0.9*4GB=3.6GB are >> used. >> >> Now let's create a new file t1, of size 512MB: >> $ dd bs=1m count=512 if=/dev/zero of=t1 >> 536870912 bytes transferred in 3.458 secs (155210354 bytes/sec) >> >> t1 should be in the hot queue now. It can be read at 2GB/s (way faster >> than disk), so at least we can be sure it is in *some* queue: >> $ dd bs=1m if=t1 of=/dev/null >> 536870912 bytes transferred in 0.263 secs (2036532935 bytes/sec) >> >> Now let's fill the hot and cold queues with something else: >> $ dd bs=1m count=16384 if=/dev/zero of=t2 >> 17179869184 bytes transferred in 57.210 secs (300290968 bytes/sec) >> >> This should have moved t1 first from the warm to the cold queue and >> then removed it from the cold queue. But strangely it can still be >> read at 2GB/s: >> $ dd bs=1m if=t1 of=/dev/null >> 536870912 bytes transferred in 0.251 secs (2135681199 bytes/sec) >> >> Why is this? How come t1 is still in the cache? >> > > Because it is more complicated. Because buffers are flipped from DMA mem > high mem in some situations. IIRC if the DMA hot queue is full the tail > buffers are flipped. Also just doing one dd is not enough to move buffers > between queues. For that they need to be read multiple times. Also write > buffers are never put in high mem or actually flipped down when written. > > In short the buffer cache is a complex beast and the few statistic numbers > systat are not enough to correctly understand the various states buffers > can be in. > > -- > :wq Claudio >
