[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [PATCH v2 0/9] hw, ui, virtfs-proxy-helper: Reduce QEMU .data/.rodat

From: Eric Blake
Subject: Re: [PATCH v2 0/9] hw, ui, virtfs-proxy-helper: Reduce QEMU .data/.rodata/.bss footprint
Date: Thu, 5 Mar 2020 08:34:25 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0

On 3/5/20 7:56 AM, Philippe Mathieu-Daudé wrote:
On 3/5/20 2:42 PM, Daniel P. Berrangé wrote:
On Thu, Mar 05, 2020 at 01:45:16PM +0100, Philippe Mathieu-Daudé wrote:
Since v1:
- merged 2 series
- reworked hw/usb/quirks
- added R-b/A-b tags

This series reduce the footprint of the QEMU binary:
.bss: 106KiB (moved to .heap)

Did this actually have an impact on the binary size, or just on the
size the elf-dissector reports ?  I'm not very familiar with ELF,
but Wikipedia's description of BSS makes me question it...

   "Typically only the length of the bss section, but no data,
    is stored in the object file. The program loader allocates
    memory for the bss section when it loads the program. On
    some platforms, some or all of the bss section is initialized
    to zeroes. Unix-like systems and Windows initialize the bss
    section to zero"

This suggests .bss has no on-disk overhead, only runtime overhead,
which is presumably going to be the same with heap allocations.

IIUC when stored in the .bss, the buffer are always allocated in memory, even if not used. By moving them to the .heap, we only allocate them when using either the adlib audio device or curses.

Virtual memory is cheap on 64-bit platforms. Just because the address range is reserved does not actually change the amount of memory in use by the machine, if the application does not touch those virtual addresses. But you do have a potential point on a 32-bit platform, where a heap allocation only when needed may avoid address space exhaustion for other cases.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

reply via email to

[Prev in Thread] Current Thread [Next in Thread]