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Date: Sat, 23 May 2015 02:22:28 +0200
From: magnum <john.magnum@...hmail.com>
To: john-dev@...ts.openwall.com
Subject: Re: autotune_run problem
On 2015-05-23 00:44, Agnieszka Bielec wrote:
> 2015-05-23 0:18 GMT+02:00 magnum <john.magnum@...hmail.com>:
>> So I'm sorry but you simply have no option other than rewriting your kernel
>> to a split, or loop, kernel. It might be hard to do this for Pomelo but not
>> impossible - and there is no alternative.
>
> ok, thanks
Actually there might obviously be an alternative though I'm not sure
there's much hope in the Pomelo case: In case your current kernel has
some bottleneck than can be removed by rewriting code or handling memory
accesses differently, it might "take off" and get significanty faster.
When that happens, you may of course increase GWS and that will likely
gain even more speed (often 2x GWS means 2x speed within a range). This
positive spiral can lead to huge boosts.
My first RAR kernel (well it was my first OpenCL code at all) was
awfully slow. This is basically a very simple format, just 50,000
iterations of SHA-1 (250,000 "updates", not 250,000 blocks) so I knew
the theorietical ideal speed should be like 15-20K c/s (about 2G SHA1
blocks per second with the devices used at the time). Due to some very
GPU-unfriendly byte shuffling within the loop, it just didn't take off
and I only got a couple thousand c/s, if that. Because of too long
durations I had to use a low GWS and because of low GWS I got even worse
speed. The negative spiral. This was very similar to where you are now.
Once I made it a "split kernel" (this alone allowing higher GWS) and
also finally realized how to optimize the byte-addressed stuff out of
the hottest loop, it "took off" like mad and I got that positive spiral:
It got faster, so I could bump GWS. That made faster yet, so I could
bump GWS even more. And so on. This is where you want to be :-)
magnum
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