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Re: Persistent malloc
- From: Carlos O'Donell <carlos at redhat dot com>
- To: Adrian May <adrian dot alexander dot may at gmail dot com>, libc-help at sourceware dot org
- Date: Tue, 7 Nov 2017 12:25:16 -0800
- Subject: Re: Persistent malloc
- Authentication-results: sourceware.org; auth=none
- References: <CAD-UbzHRNzchaBNVR67v1NUAi25h09_9291THRto_iyBRSEYDg@mail.gmail.com>
On 11/07/2017 11:56 AM, Adrian May wrote:
> It worked fine until I tried it from multiple threads, at which point I
> fell foul of the arena-per-thread thing.
> It's not obvious to me from the sources how I could switch that off. I saw
> tunables called "arena test" and "arena max" but I'm not sure what they do.
> In _libc__malloc the per-thread fetching of the arena looks hard wired as a
> macro. I was hoping not to have to recompile libc incidentally.
man 3 mallopt
Gives you one arena, and all threads use that arena (main arena).
> Then again, what I really want is a pmalloc and pfree function so I can
> specifically request persistent allocation but default to the existing
> volatile per-thread stuff.
You can have whatever you are willing to design, implement, champion, and
support... this is an open source project driven by consensus :-)
> Could I just declare an empty malloc_state and pass it to int_malloc from
> pmalloc? Trouble is, in sysmalloc (dunno if that's relevant or not) I see
> (av != &main_arena) choosing whether or not to call MORECORE at all.
Only the main arena (arena 0) uses process heap, all other arenas use mmap.
> Is there any way to unravel these hard coded bits? Could I arrange for only
> pmalloc to use the main arena while the main thread has its own per-thread
> arena? Or is there some different pre-existing concept for how this kind of
> thing should be done?
You could arrange for a pmalloc to take chunks from the main arena only.
Such a thread would always have it's own thread-local cache (tcache) if you
don't bypass it.
It's hard to know what you want without some clear design document.