On Tue, Jun 27, 2023 at 01:13:25AM +0100, Matthew Wilcox wrote:
> 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?
Oh.
for (i = 0; ....) {
bh = __find_get_block(bdev, 1, 512);
That's a '1' being passed to __find_get_block, not 'i'.
/me goes and gets more coffee.
Maybe it's CONFIG_PREEMPT_RT=y doing something to the locks that
isn't obvious here...
-Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
