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Subject: Re: [PATCH] aio: Convert ioctx_table to XArray
To:     Matthew Wilcox <willy@infradead.org>
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>,
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        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Dan Carpenter <dan.carpenter@oracle.com>,
        kent.overstreet@gmail.com
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From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <d45b92ea-ee57-0228-a23f-419b5b72ed59@kernel.dk>
Date:   Tue, 11 Dec 2018 11:52:05 -0700
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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