From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Subject: Re: [PATCH] aio: Convert ioctx_table to XArray To: Matthew Wilcox Cc: Jeff Moyer , Alexander Viro , Benjamin LaHaise , Andrew Morton , Kees Cook , linux-fsdevel@vger.kernel.org, linux-aio@kvack.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Dan Carpenter , kent.overstreet@gmail.com References: <20181128183531.5139-1-willy@infradead.org> <20181211175156.GF6830@bombadil.infradead.org> <0f77a532-0d88-78bc-b9cc-06bb203a0405@kernel.dk> <6b9a45c4-47a2-4c44-aa7e-6e5e90eff9df@kernel.dk> <20181211185105.GI6830@bombadil.infradead.org> From: Jens Axboe Message-ID: Date: Tue, 11 Dec 2018 11:52:05 -0700 MIME-Version: 1.0 In-Reply-To: <20181211185105.GI6830@bombadil.infradead.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: On 12/11/18 11:51 AM, Matthew Wilcox wrote: > On Tue, Dec 11, 2018 at 11:32:54AM -0700, Jens Axboe wrote: >> Don't see any regressions. But if we're fiddling with it anyway, can't >> we do something smarter? Make the fast path just index a table, and put >> all the big hammers in setup/destroy. We're spending a non-substantial >> amount of time doing lookups, that's really no different before and >> after the patch. > > Thanks for checking it out. > > I think the fast path does just index a table. Until you have more than > 64 pointers in the XArray, it's just xa->head->slots[i]. And then up > to 4096 pointers, it's xa->head->slots[i >> 6]->slots[i]. It has the > advantage that if you only have one kioctx (which is surely many programs > using AIO), it's just xa->head, so even better than a table lookup. > > It'll start to deteriorate after 4096 kioctxs, with one extra indirection > for every 6 bits, but by that point, we'd've been straining the memory > allocator to allocate a large table anyway. I agree, and nobody cares about 4k kioctxs, you're way into the weeds at that point anyway. So as the followup said, I think we're fine as-is for this particular case. -- Jens Axboe