linux-kernel - Re: [PATCH v5 1/3] riscv: optimized memcpy

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Message-ID: <20211001112354.GA10720@duo.ucw.cz>
Date:   Fri, 1 Oct 2021 13:23:54 +0200
From:   Pavel Machek <pavel@....cz>
To:     Matteo Croce <mcroce@...ux.microsoft.com>
Cc:     linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Atish Patra <atish.patra@....com>,
        Emil Renner Berthing <kernel@...il.dk>,
        Akira Tsukamoto <akira.tsukamoto@...il.com>,
        Drew Fustini <drew@...gleboard.org>,
        Bin Meng <bmeng.cn@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Guo Ren <guoren@...nel.org>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v5 1/3] riscv: optimized memcpy

Hi!

> From: Matteo Croce <mcroce@...rosoft.com>
> 
> Write a C version of memcpy() which uses the biggest data size allowed,
> without generating unaligned accesses.
> 
> The procedure is made of three steps:
> First copy data one byte at time until the destination buffer is aligned
> to a long boundary.
> Then copy the data one long at time shifting the current and the next u8
> to compose a long at every cycle.
> Finally, copy the remainder one byte at time.
> 
> On a BeagleV, the TCP RX throughput increased by 45%:
> 
> before:
> 
> $ iperf3 -c beaglev
> Connecting to host beaglev, port 5201
> [  5] local 192.168.85.6 port 44840 connected to 192.168.85.48 port 5201
> [ ID] Interval           Transfer     Bitrate         Retr  Cwnd
> [  5]   0.00-1.00   sec  76.4 MBytes   641 Mbits/sec   27    624 KBytes
> [  5]   1.00-2.00   sec  72.5 MBytes   608 Mbits/sec    0    708 KBytes
> 
> after:
> 
> $ iperf3 -c beaglev
> Connecting to host beaglev, port 5201
> [  5] local 192.168.85.6 port 44864 connected to 192.168.85.48 port 5201
> [ ID] Interval           Transfer     Bitrate         Retr  Cwnd
> [  5]   0.00-1.00   sec   109 MBytes   912 Mbits/sec   48    559 KBytes
> [  5]   1.00-2.00   sec   108 MBytes   902 Mbits/sec    0    690
> KBytes

That's really quite cool. Could you see if it is your "optimized
unaligned" copy doing the difference?>

+/* convenience union to avoid cast between different pointer types */
> +union types {
> +	u8 *as_u8;
> +	unsigned long *as_ulong;
> +	uintptr_t as_uptr;
> +};
> +
> +union const_types {
> +	const u8 *as_u8;
> +	unsigned long *as_ulong;
> +	uintptr_t as_uptr;
> +};

Missing consts here?

Plus... this is really "interesting" coding style. I'd just use casts
in kernel.

Regards,						Pavel

-- 
http://www.livejournal.com/~pavelmachek

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