Re: [PATCH] fs/buffer.c: remove per-CPU buffer_head lookup cache

[Matthew Wilcox <willy@infradead.org>]

On Tue, Jun 27, 2023 at 09:30:09AM +1000, Dave Chinner wrote:
> On Mon, Jun 26, 2023 at 07:47:42PM +0100, Matthew Wilcox wrote:
> > On Mon, Jun 26, 2023 at 03:04:53PM -0300, Marcelo Tosatti wrote:
> > > Upon closer investigation, it was found that in current codebase, lookup_bh_lru
> > > is slower than __find_get_block_slow:
> > > 
> > >  114 ns per __find_get_block
> > >  68 ns per __find_get_block_slow
> > > 
> > > So remove the per-CPU buffer_head caching.
> > 
> > LOL.  That's amazing.  I can't even see why it's so expensive.  The
> > local_irq_disable(), perhaps?  Your test case is the best possible
> > one for lookup_bh_lru() where you're not even doing the copy.
> 
> I think it's even simpler than that.
> 
> i.e. the lookaside cache is being missed, so it's a pure cost and
> the code is always having to call __find_get_block_slow() anyway.

How does that happen?

__find_get_block(struct block_device *bdev, sector_t block, unsigned size)
{
        struct buffer_head *bh = lookup_bh_lru(bdev, block, size);

        if (bh == NULL) {
                /* __find_get_block_slow will mark the page accessed */
                bh = __find_get_block_slow(bdev, block);
                if (bh)
                        bh_lru_install(bh);

The second (and all subsequent) calls to __find_get_block() should find
the BH in the LRU.

> IMO, this is an example of how lookaside caches are only a benefit
> if the working set of items largely fits in the lookaside cache and
> the cache lookup itself is much, much slower than a lookaside cache
> miss.

But the test code he posted always asks for the same buffer each time.
So it should find it in the lookaside cache?