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Re: [Qemu-devel] [PATCH v18 05/10] xbitmap: add more operations

From: Matthew Wilcox
Subject: Re: [Qemu-devel] [PATCH v18 05/10] xbitmap: add more operations
Date: Thu, 30 Nov 2017 06:39:52 -0800
User-agent: Mutt/1.9.1 (2017-09-22)

On Thu, Nov 30, 2017 at 10:35:03PM +0900, Tetsuo Handa wrote:
> According to xb_set_bit(), it seems to me that we are trying to avoid memory 
> allocation
> for "struct ida_bitmap" when all set bits within a 1024-bits bitmap reside in 
> the first
> 61 bits.
> But does such saving help? Is there characteristic bias that majority of set 
> bits resides
> in the first 61 bits, for "bit" is "unsigned long" which holds a page number 
> (isn't it)?
> If no such bias, wouldn't eliminating radix_tree_exception() case and always 
> storing
> "struct ida_bitmap" simplifies the code (and make the processing faster)?

It happens all the time.  The vast majority of users of the IDA set
low bits.  Also, it's the first 62 bits -- going up to 63 bits with the
XArray rewrite.

I do plan to redo the xbitmap on top of the XArray; I'm just trying to
get the XArray merged first.  The IDA and xbitmap code will share much
more code when that happens.

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