Re: improving malloc

[KOSAKI Motohiro <kosaki dot motohiro at gmail dot com>]

On Sun, Jan 6, 2013 at 8:45 AM, Rich Felker <dalias@aerifal.cx> wrote:
> On Sun, Jan 06, 2013 at 10:56:18AM +0100, OndÅej BÃlka wrote:
>> > Other considerations are memory fragmentation, how quickly
>> > it can give back unused memory to the OS, etc. etc.
>> >
>> For giving memory back to OS when linux gets volatile ranges then
>> we can finally do not have to defer returning memory because zeroing
>> pages is expensive.
>> I wanted to suggest at linux-kernel to keep pages returned to linux
>> at linked list and for allocations prefer these as they do not have
>> to be zeroed.
>
> Preferring them is backwards; it will cause more page faults and use
> more memory. Once you've returned memory to the kernel, you should
> avoid using it again unless absolutely necessary. It's _possible_
> that, for some usage cases, calloc would want to use this memory, but
> for malloc it's always a pessimization.

Hi

I'm one of kernel memory folks and I'd like to explain how mvolatile() does.
It give a hint that given ranges are discardable to kernel. Thus when getting
memory pressure, kernel just drop such memory instead of swap out. It help
to minimalize wrong returning memory cost.

see https://lwn.net/Articles/531305/
      http://lwn.net/Articles/518130/

However, of course, we have no free lunch. 1) minor page fault is
still expensive for allocator
fast path 2) mvolatile is less multi thread friendly than current
madvise(MADV_DONTNEED).
Because madvise(MADV_DONTNEED) don't need to take mmap_sem in kernel and it has
excellent multi threaded performance when no wrong memory returning case.

I hope mvolatile finally replaces MADV_DONTNEED, however, It need
carefully eveluation.
btw, I don't understand Ondrej's "linked list" is which mechanism
point to. Can anyone clarify?