Hi Nhat, On Wed, May 3, 2023 at 3:38 AM Nhat Pham <nphamcs@xxxxxxxxx> wrote: > There is currently no good way to query the page cache state of large > file sets and directory trees. There is mincore(), but it scales poorly: > the kernel writes out a lot of bitmap data that userspace has to > aggregate, when the user really doesn not care about per-page > information in that case. The user also needs to mmap and unmap each > file as it goes along, which can be quite slow as well. > > Some use cases where this information could come in handy: > * Allowing database to decide whether to perform an index scan or > direct table queries based on the in-memory cache state of the > index. > * Visibility into the writeback algorithm, for performance issues > diagnostic. > * Workload-aware writeback pacing: estimating IO fulfilled by page > cache (and IO to be done) within a range of a file, allowing for > more frequent syncing when and where there is IO capacity, and > batching when there is not. > * Computing memory usage of large files/directory trees, analogous to > the du tool for disk usage. > > More information about these use cases could be found in the following > thread: > > https://lore.kernel.org/lkml/20230315170934.GA97793@xxxxxxxxxxx/ > > This patch implements a new syscall that queries cache state of a file > and summarizes the number of cached pages, number of dirty pages, number > of pages marked for writeback, number of (recently) evicted pages, etc. > in a given range. Currently, the syscall is only wired in for x86 > architecture. > > NAME > cachestat - query the page cache statistics of a file. > > SYNOPSIS > #include <sys/mman.h> > > struct cachestat_range { > __u64 off; > __u64 len; > }; > > struct cachestat { > __u64 nr_cache; > __u64 nr_dirty; > __u64 nr_writeback; > __u64 nr_evicted; > __u64 nr_recently_evicted; > }; > > int cachestat(unsigned int fd, struct cachestat_range *cstat_range, > struct cachestat *cstat, unsigned int flags); > > DESCRIPTION > cachestat() queries the number of cached pages, number of dirty > pages, number of pages marked for writeback, number of evicted > pages, number of recently evicted pages, in the bytes range given by > `off` and `len`. > > An evicted page is a page that is previously in the page cache but > has been evicted since. A page is recently evicted if its last > eviction was recent enough that its reentry to the cache would > indicate that it is actively being used by the system, and that > there is memory pressure on the system. > > These values are returned in a cachestat struct, whose address is > given by the `cstat` argument. > > The `off` and `len` arguments must be non-negative integers. If > `len` > 0, the queried range is [`off`, `off` + `len`]. If `len` == > 0, we will query in the range from `off` to the end of the file. > > The `flags` argument is unused for now, but is included for future > extensibility. User should pass 0 (i.e no flag specified). > > Currently, hugetlbfs is not supported. > > Because the status of a page can change after cachestat() checks it > but before it returns to the application, the returned values may > contain stale information. > > RETURN VALUE > On success, cachestat returns 0. On error, -1 is returned, and errno > is set to indicate the error. > > ERRORS > EFAULT cstat or cstat_args points to an invalid address. > > EINVAL invalid flags. > > EBADF invalid file descriptor. > > EOPNOTSUPP file descriptor is of a hugetlbfs file > > Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx> > --- > arch/x86/entry/syscalls/syscall_32.tbl | 1 + > arch/x86/entry/syscalls/syscall_64.tbl | 1 + This should be wired up on each and every architecture. Currently we're getting <stdin>:1567:2: warning: #warning syscall cachestat not implemented [-Wcpp] in linux-next for all the missing architectures. Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds
