Messages in this thread | | | Date | Thu, 7 Oct 2021 21:55:21 +0100 | From | Matthew Wilcox <> | Subject | Re: [PATCH v2] mm: Optimise put_pages_list() |
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On Thu, Oct 07, 2021 at 12:31:09PM -0700, Andrew Morton wrote: > On Thu, 7 Oct 2021 20:21:37 +0100 "Matthew Wilcox (Oracle)" <willy@infradead.org> wrote: > > > Instead of calling put_page() one page at a time, pop pages off > > the list if their refcount was too high and pass the remainder to > > put_unref_page_list(). This should be a speed improvement, but I have > > no measurements to support that. Current callers do not care about > > performance, but I hope to add some which do. > > Don't you think it would actually be slower to take an additional pass > across the list? If the list is long enough to cause cache thrashing. > Maybe it's faster for small lists.
My first response is an appeal to authority -- release_pages() does this same thing. Only it takes an array, constructs a list and passes that to put_unref_page_list(). So if that's slower (and lists _are_ slower than arrays), we should have a put_unref_page_array().
Second, we can follow through the code paths and reason about it.
Before:
while (!list_empty(pages)) { put_page(victim); page = compound_head(page); if (put_page_testzero(page)) __put_page(page); __put_single_page(page) __page_cache_release(page); mem_cgroup_uncharge(page); <--- free_unref_page(page, 0); free_unref_page_prepare() local_lock_irqsave(&pagesets.lock, flags); free_unref_page_commit(page, pfn, migratetype, order); local_unlock_irqrestore(&pagesets.lock, flags);
After:
free_unref_page_list(pages); list_for_each_entry_safe(page, next, list, lru) { if (!free_unref_page_prepare(page, pfn, 0)) { }
local_lock_irqsave(&pagesets.lock, flags); list_for_each_entry_safe(page, next, list, lru) { free_unref_page_commit() } local_unlock_irqrestore(&pagesets.lock, flags);
So the major win here is that we disable/enable interrupts once per batch rather than once per page.
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