On Sun, Jul 05, 2020 at 09:25:39AM +0200, Jan Ziak wrote:
On Sun, Jul 5, 2020 at 8:32 AM Andreas Dilger adilger@dilger.ca wrote:
On Jul 4, 2020, at 8:46 PM, Jan Ziak 0xe2.0x9a.0x9b@gmail.com wrote:
On Sun, Jul 5, 2020 at 4:16 AM Matthew Wilcox willy@infradead.org wrote:
On Sun, Jul 05, 2020 at 04:06:22AM +0200, Jan Ziak wrote:
Hello
At first, I thought that the proposed system call is capable of reading *multiple* small files using a single system call - which would help increase HDD/SSD queue utilization and increase IOPS (I/O operations per second) - but that isn't the case and the proposed system call can read just a single file.
Without the ability to read multiple small files using a single system call, it is impossible to increase IOPS (unless an application is using multiple reader threads or somehow instructs the kernel to prefetch multiple files into memory).
What API would you use for this?
ssize_t readfiles(int dfd, char **files, void **bufs, size_t *lens);
I pretty much hate this interface, so I hope you have something better in mind.
I am proposing the following:
struct readfile_t { int dirfd; const char *pathname; void *buf; size_t count; int flags; ssize_t retval; // set by kernel int reserved; // not used by kernel };
If you are going to pass a struct from userspace to the kernel, it should not mix int and pointer types (which may be 64-bit values, so that there are not structure packing issues, like:
struct readfile { int dirfd; int flags; const char *pathname; void *buf; size_t count; ssize_t retval; };
It would be better if "retval" was returned in "count", so that the structure fits nicely into 32 bytes on a 64-bit system, instead of being 40 bytes per entry, which adds up over many entries, like.
I know what you mean and it is a valid point, but in my opinion it shouldn't (in most cases) be left to the programmer to decide what the binary layout of a data structure is - instead it should be left to an optimizing compiler to decide it.
We don't get that luxury when creating user/kernel apis in C, sorry.
I suggest using the pahole tool if you are interested in seeing the "best" way a structure can be layed out, it can perform that optimization for you so that you know how to fix your code.
thanks,
greg k-h