Messages in this thread | | | Date | Tue, 27 Jun 2023 01:13:25 +0100 | From | Matthew Wilcox <> | Subject | Re: [PATCH] fs/buffer.c: remove per-CPU buffer_head lookup cache |
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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?
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