linux-kernel - Re: [PATCH] fdtable: Eradicate fdarray overflow.

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Message-Id: <200610112307.38485.vlobanov@speakeasy.net>
Date:	Wed, 11 Oct 2006 23:07:38 -0700
From:	Vadim Lobanov <vlobanov@...akeasy.net>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	akpm@...l.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fdtable: Eradicate fdarray overflow.

On Wednesday 11 October 2006 22:19, Eric Dumazet wrote:
> Hi Vadim
>
> I find your PAGE_SIZE/4 minimum allocation quite unjustified.
>
> For architectures with 64K PAGE_SIZE, we endup allocating 16K, for poor
> tasks that happen to touch a not so high (>= 64) file descriptor...
>
> I would vote for a fixed size, like 1024

In my opinion, always picking 1024 would be highly suboptimal for some 
architectures (x86-64 in particular -- that's a whole page, just for the 
fdarray!). If anything, I'd prefer something similar to this pseudo-code:

#define FDTABLE_MIN min_t(uint, PAGE_SIZE / 4 / sizeof(struct file *), 1024)
...
nr /= FDTABLE_MIN;
nr = roundup_pow_of_two(nr + 1);
nr *= FDTABLE_MIN;

gcc should be smart enough to optimize that expression into a single constant. 
At least it did (version 4.1.0) in my quick test here.

> Eric

Let me know what you think. Please don't just go radio-silent on me. ;)

-- Vadim Lobanov
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