Re: [RFC v2] nvfs: a filesystem for persistent memory

Mikulas Patocka
2021-01-10T21:16:56Z

On Sun, 10 Jan 2021, Al Viro wrote:

> On Thu, Jan 07, 2021 at 08:15:41AM -0500, Mikulas Patocka wrote:
> > Hi
> >
> > I announce a new version of NVFS - a filesystem for persistent memory.
> > http://people.redhat.com/~mpatocka/nvfs/
> Utilities, AFAICS
>
> > git://leontynka.twibright.com/nvfs.git
> Seems to hang on git pull at the moment... Do you have it anywhere else?

I saw some errors 'git-daemon: fatal: the remote end hung up unexpectedly'
in syslog. I don't know what's causing them.

> > I found out that on NVFS, reading a file with the read method has 10%
> > better performance than the read_iter method. The benchmark just reads the
> > same 4k page over and over again - and the cost of creating and parsing
> > the kiocb and iov_iter structures is just that high.
>
> Apples and oranges... What happens if you take
>
> ssize_t read_iter_locked(struct file *file, struct iov_iter *to, loff_t *ppos)
> {
> struct inode *inode = file_inode(file);
> struct nvfs_memory_inode *nmi = i_to_nmi(inode);
> struct nvfs_superblock *nvs = inode->i_sb->s_fs_info;
> ssize_t total = 0;
> loff_t pos = *ppos;
> int r;
> int shift = nvs->log2_page_size;
> size_t i_size;
>
> i_size = inode->i_size;
> if (pos >= i_size)
> return 0;
> iov_iter_truncate(to, i_size - pos);
>
> while (iov_iter_count(to)) {
> void *blk, *ptr;
> size_t page_mask = (1UL << shift) - 1;
> unsigned page_offset = pos & page_mask;
> unsigned prealloc = (iov_iter_count(to) + page_mask) >> shift;
> unsigned size;
>
> blk = nvfs_bmap(nmi, pos >> shift, &prealloc, NULL, NULL, NULL);
> if (unlikely(IS_ERR(blk))) {
> r = PTR_ERR(blk);
> goto ret_r;
> }
> size = ((size_t)prealloc << shift) - page_offset;
> ptr = blk + page_offset;
> if (unlikely(!blk)) {
> size = min(size, (unsigned)PAGE_SIZE);
> ptr = empty_zero_page;
> }
> size = copy_to_iter(to, ptr, size);
> if (unlikely(!size)) {
> r = -EFAULT;
> goto ret_r;
> }
>
> pos += size;
> total += size;
> } while (iov_iter_count(to));
>
> r = 0;
>
> ret_r:
> *ppos = pos;
>
> if (file)
> file_accessed(file);
>
> return total ? total : r;
> }
>
> and use that instead of your nvfs_rw_iter_locked() in your
> ->read_iter() for DAX read case? Then the same with
> s/copy_to_iter/_copy_to_iter/, to see how much of that is
> "hardening" overhead.
>
> Incidentally, what's the point of sharing nvfs_rw_iter() for
> read and write cases? They have practically no overlap -
> count the lines common for wr and !wr cases. And if you
> do the same in nvfs_rw_iter_locked(), you'll see that the
> shared parts _there_ are bloody pointless on the read side.

That's a good point. I split nvfs_rw_iter to separate functions
nvfs_read_iter and nvfs_write_iter - and inlined nvfs_rw_iter_locked into
both of them. It improved performance by 1.3%.

> Not that it had been more useful on the write side, really,
> but that's another story (nvfs_write_pages() handling of
> copyin is... interesting). Let's figure out what's going
> on with the read overhead first...
>
> lib/iov_iter.c primitives certainly could use massage for
> better code generation, but let's find out how much of the
> PITA is due to those and how much comes from you fighing
> the damn thing instead of using it sanely...

The results are:

read: 6.744s
read_iter: 7.417s
read_iter - separate read and write path: 7.321s
Al's read_iter: 7.182s
Al's read_iter with _copy_to_iter: 7.181s

Mikulas