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

[Mikulas Patocka]

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