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Date:   Tue, 11 Dec 2018 10:51:05 -0800
From:   Matthew Wilcox <willy@infradead.org>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Jeff Moyer <jmoyer@redhat.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Benjamin LaHaise <bcrl@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        linux-fsdevel@vger.kernel.org, linux-aio@kvack.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Dan Carpenter <dan.carpenter@oracle.com>,
        kent.overstreet@gmail.com
Subject: Re: [PATCH] aio: Convert ioctx_table to XArray
Message-ID: <20181211185105.GI6830@bombadil.infradead.org>
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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.