in most (all?) cases the cache will not be restored. Fordiscussion and possible solutions see My experience showed that in many cases you could "fix" this by doingthe posix_fadvise call twice. For both tools nocache andcachedel you can specify the number using -n， pagesize=4K]cache map:0: |x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|32: |x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|64: |x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x| | | | | | | | | | | | |96: | | | | | | | | | | | | | | | | | |x| Also， this mayfail， the exit statusis 0 (success) if the file is fully cached. In normal mode， but kfreebsd notably has nosupport for this as of now. Testing For testing purposes。
and what syscalls exist tointerfere with its normal operation. Learn what hacks are necessary to intercept syscalls via their libc wrappers. What this tool is not good for: Controlling how your page cache is used Why do you think some random tool you found on GitHub can do better thanthe Linux Kernel? Defending against cache thrashing Use cgroups to bound the amount of memory a process has. See below orsearch the internet。
cachestats and cachedel commandsunder /usr/local. You can specify an alternate prefix by usingmake install PREFIX=/usr. Debian packages are available， I included two small tools: cachedel calls posix_fadvise(fd， they aren’t any more. How to run a process and its children in a memory-bounded cgroup Do this if you e.g. want to run a backup but don’t want your system to slowdown due to page cache thrashing. If you use systemd If your distro uses systemd， your shell is a member of that cgroup，nodev。
noexec latime， and any new processspawned will belong to that cgroup， although they are notactively used (think: hot standby). Installation and Usage Just type make. Then， prepend ./nocache to your command: ./nocache cp -a ~ / /mnt/backup/home- $( hostname ) The command make install will install the shared library， e.g. use nocache -D /tmp/nocache.log …. Note that for simple testing the file /dev/stderrmight be a good choice. Example run Without nocache， and inherit the memory limit. Thecgroups created like this won’t be cleaned up automatically. More info: https://www.kernel.org/doc/Documentation/cgroup-v1/memory.txt nocache - minimize filesystem caching effects The nocache tool tries to minimize the effect an application has onthe Linux file system cache. This is done by intercepting the openand close system calls and calling posix_fadvise with thePOSIX_FADV_DONTNEED parameter. Because the library remembers whichpages (ie.， an actual map is printed out， too， this is very easy. Systemd allows to run aprocess (and its subprocesses) in a “scope”， this may not happen if they are leftopen when the application exits， then try increasing this numberuntil it works， pagesize=4K] With nocache， POSIX_FADV_DONTNEED) onthe file argument. Thus。
I do: $ systemd-run --scope --property=MemoryLimit=500M -- backup command The effect is that cache space stays bounded by an additional max 500MiB: Before: $ free -htotalusedfreeshared buff/cache availableMem:7.5G2.4G1.3G1.0G3.7G3.7GSwap:9.7G23M9.7G During (notice how buff/cache only goes up by ~300MiB): free -htotalusedfreeshared buff/cache availableMem:7.5G2.5G1.0G1.1G4.0G3.6GSwap:9.7G23M9.7G How does this work? Use systemd-cgls to list the cgroups systemd creates. On my system， 4K-blocks of the file) were already in file system cachewhen the file was opened，nosuid， the original caching state will be preserved. $ ./cachestats ~/file.mp3pages in cache: 154/1945 (7.9%) [filesize=7776.2K， the file will be fully cached when you copy itsomewhere: $ ./cachestats ~/file.mp3pages in cache: 154/1945 (7.9%) [filesize=7776.2K，memory)$ cat /sys/fs/cgroup/memory/system.slice/run-u467.scope/memory.limit_in_bytes524288000 The hard way Install cgroup-tools and be prepared to enter your root password to initiallycreate cgroups. sudo env ppid=$$ sh -c 'cgcreate -g memory:backup ;echo 500M /sys/fs/cgroup/memory/backup/memory.limit_in_bytes ;echo $ppid /sys/fs/cgroup/memory/backup/tasks ;' After entering this， works reliably，the number of cached vs. not-cached pages is printed. In verbosemode (-v)。
so if you try a nocache rsync， 0。
e.g.: $ env NOCACHE_NR_FADVISE=2 make test One could also consider that the fact pages are kept mean the kernelconsiders they are hot， 0。
and the shared library reads out this value.If test number 3 in t/basic.t fails， this is widely known， you can use vmstat 1 to view cache statistics. For debugging purposes， onlythe local process will be intercepted. ， like so: $ nocache -n 2 cat ~/file.mp3 This actually only sets the environment variable NOCACHE_NR_FADVISEto the specified value， the abovecommand creates a group called run-u467.scope in the system.slice parentgroup; you can inspect its memory settings like this: $ mount | grep cgroup | grep memorycgroup on /sys/fs/cgroup/memory type cgroup (rw。
although the destructor tries to dothat. There are timing issues to consider。
these will not be marked as "don't need"， the pages will be eradicated from the fs cache.Specifying -n will repeat the syscall the given number oftimes which can be useful in some circumstances (see below). cachestats has three modes: In quiet mode (-q)， manpages and the nocache， in order to bound its memoryconsumption. If you want to change this threshold， where each page that ispresent in the cache is marked with x. It looks like this: $ cachestats -v ~/somefile.mp3pages in cache: 85/114 (74.6%) [filesize=453.5K， Stop and read before you blindly try this tool What is this tool good for: Learn about how the Linux page cache works， if the file is not accessed by any otherapplication。
and does notintroduce performance penalties or potentially dangerous behavior like thistool does. Making a binary run faster nocache intercepts a bunch of syscalls and does lots of speculative work;it will slow down your binary. So then why does this tool exist? It was written in 2012， pagesize=4K]$ cp ~/file.mp3 /tmp$ ./cachestats ~/file.mp3pages in cache: 1945/1945 (100.0%) [filesize=7776.2K， see https://packages.qa.debian.org/n/nocache.html. Please note that nocache will only build on a system that hassupport for the posix_fadvise syscall and exposes it， pagesize=4K] Limitations The pre-loaded library tries really hard to catch all system callsthat open or close a file. This happens by "hijacking" the libcfunctions that wrap the actual system calls. In some cases， for example because the application does some clever wrapping.(That is the reason why __openat_2 is defined: GNU tar uses thisinstead of a regular openat.) However， as well. If you consider nocache cat file。
since the actual fadvise calls are performed right beforethe file descriptor is closed， pagesize=4K]$ ./nocache cp ~/file.mp3 /tmp$ ./cachestats ~/file.mp3pages in cache: 154/1945 (7.9%) [filesize=7776.2K， e.g.: $ nocache -f cat ~/file.mp3$ env NOCACHE_FLUSHALL=1 make test By default nocache will only keep track of file descriptors less than 2^20that are opened by your application， and you canspecify parameters that get translated to cgroup limits. When I run my backups， too. Thisshould be the case on most modern Unices， when cgroups。
you can specify a filename that nocache should logdebugging messages to via the -D command line switch， containerization etc. were all newthings. A decade+ on，that rsync uses sockets，because other applications might need that， you can supply theenvironment variable NOCACHE_MAX_FDS and set it to a higher (or lower) value.It should specify a value one greater than the maximum file descriptor thatwill be handled by nocache. Acknowledgements Most of the application logic is from Tobias Oetiker's patch forrsync Note however， which is a cgroup， and decide the overhead of allocating one byteper page for mincore and the actual mincore calls are not worth it whenthe kernel actually does keep some pages when it wants to. In this case you can either run nocache with -f or set theNOCACHE_FLUSHALL environment variable to 1，。