linux-kernel - Re: [PATCH 1/6] Adjust maximum MSIZE to account for p9 header

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Date:   Sun, 18 Dec 2022 13:52:55 -0600
From:   evanhensbergen@...oud.com
To:     asmadeus@...ewreck.org
Cc:     Ron Minnich <rminnich@...il.com>,
        Eric Van Hensbergen <ericvh@...il.com>,
        Christian Schoenebeck <linux_oss@...debyte.com>,
        v9fs-developer@...ts.sourceforge.net,
        Latchesar Ionkov <lucho@...kov.net>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/6] Adjust maximum MSIZE to account for p9 header

Oh crap - I’m such an idiot - yeah IOHDRSZ was what I meant to use in the first place.  Although I am left wondering why IOHDRSZ is 24 - It does seem to be one extra byte, but I won’t begrudge lucho for going for an even number.

I’ll fix this up before re-releasing the patchset.  I’m just finishing breaking up my write back fixes to make them a bit more consumable as a patch.

          -eric



> On Dec 18, 2022, at 1:46 PM, asmadeus@...ewreck.org wrote:
> 
> ron minnich wrote on Sun, Dec 18, 2022 at 08:50:18AM -0800:
>> it's fine. tbh, I doubt the fact that you were fetching 31 vs 32 pages
>> mattered as much as the fact that you weren't fetching *4k at a time* :-)
> 
> Yes, I think we can just blanket this as +4k and it wouldn't change
> much; I've been using 1MB+4k for rdma in previous tests...
> 
> We still aren't doing things 4k at a time with this though, I'd suggest
> rounding down the rsize > msize check in p9_client_read_once():
> 
>        if (!rsize || rsize > clnt->msize - P9_IOHDRSZ)
>                rsize = clnt->msize - P9_IOHDRSZ;
> 
> to something that's better aligned; for some reason I thought we had
> that already.  . . but thinking again the sizes are probably driven by
> the cache and will be 4k multiples already?
> 
>>> -#define DEFAULT_MSIZE (128 * 1024)
>>> +/* DEFAULT MSIZE = 32 pages worth of payload + P9_HDRSZ +
>>> + * room for write (16 extra) or read (11 extra) operands.
>>> + */
>>> +
>>> +#define DEFAULT_MSIZE ((128 * 1024) + P9_HDRSZ + 16)
> 
> There's P9_IOHDRSZ for that ;)
> 
> But I guess with the comment it doesn't matter much either way.
> 
> -- 
> Dominique