Julia: command line flag to limit heap memory usage?

[tanmaykm]

Should there be a command line flag to limit heap memory used by a Julia process?
Similar to -Xms and -Xmx for the JVM.

This may be useful while running Julia inside containers (docker) with memory limits or
running multiple Julia processes on the same machine.

[yuyichao]

What kind of memory should the options limit?

[tanmaykm]

Limit on the heap would be the most useful I suppose.
Would setrlimit of RLIMIT_AS and RLIMIT_STACK do the trick on linux?

[yuyichao]

RLIMIT_STACK doesn't affect memory usage at all, only how much recursion you can have before a stackoverflow happens. (You can have multiple stacks for example and the stack space is pretty small anyway)

RLIMIT_AS is a pretty terrible setting, it doesn't limit actually memory usage and it makes memory management harder and less efficient by not being able to reserve a large flat address space.

Limit on the heap would be the most useful I suppose.

What heap though? A GC option to do GC more aggressively when approaching a user defined memory limit is possible, although there's a lot of memory that is allocated by user/C library as well as LLVM that we can't control.

[ViralBShah]

Yes more aggressive GC when approaching a user specified limit would significantly reduce memory consumption on JuliaBox.

[ViralBShah]

Also useful for running Julia in many shared settings where otherwise we will end up consuming all memory before the GC pressure kicks in.

[nkottary]

I think the limit that we pass as command line argument should be checked here:

    /**
     * src/gc.c:826
     */

    if (__unlikely((gc_num.allocd += osize) >= 0) || gc_debug_check_pool()) {
        //gc_num.allocd -= osize;
        jl_gc_collect(0);
        //gc_num.allocd += osize;
    }

And here:

    /**
     * src/gc.c:553
     */

    #define should_collect() (__unlikely(gc_num.allocd>0))

Is that right?

[yuyichao]

No,. It should be folded to how allocd is updated.

[ViralBShah]

Based on conversation with @yuyichao, the idea is to try and add a condition to detect memory pressure in the heuristic for full collection at gc.c:1768.

        (full || large_frontier ||
        ((not_freed_enough || promoted_bytes >= gc_num.interval) &&
         (promoted_bytes >= default_collect_interval || prev_sweep_full)) ||
        gc_check_heap_size(live_sz_ub, live_sz_est)) &&
       gc_num.pause > 1

[yuyichao]

Also the decision of collection interval a few lines below (assignment to gc_num.interval)

[JeffBezanson]

We call uv_get_total_memory to determine the system memory size, which is used in turn to set the max collect interval. My impression is that the new option should manually replace the value returned by uv_get_total_memory.

We use RLIMIT_AS as a hint for how much vmem we try to map up front, but we also try smaller amounts until the mapping succeeds, so we probably don't need to change that code path.

[vtjnash]

I think we can also update uv_get_total_memory to chase the crazy linux cgroup information. My current understanding is that it needs to do the following (falling back to the current lookup if any of these steps fail):

• parse /proc/self/mountinfo to locate the cgroup mount root(s) for the memory subsystem(s)
• parse /proc/self/cgroup to find the cgroup path(s) for the memory subsystem(s) or by their names from parsing mountinfo, and determine which combination is providing the memory limit hierarchy (I think only one combination should match?)
• read <root>/<cgroup-path>/memory.limit_in_bytes for that hierarchy

[aviks]

There are requests on the mailing lists for something similar: https://groups.google.com/d/msg/julia-users/CNE3xBjpCEk/Je6zi-mxBAAJ

[joshjob42]

I'm don't think I'm knowledgeable enough about any of this to make this PR, but I just thought I'd comment to say that there are definitely still people who can't use Julia in their HPC environment because of the sorts of problems referenced in #10390 / here. If I run 'julia -p 8' on either 0.5.1 or master, on a 64GB machine, the job hangs or I get "libsuitesparseconfig.so: failed to map segment from shared object: Cannot allocate memory" or reports that the processes didn't connect to Master within 60 seconds, etc.

It was easy enough when I had to recompile from scratch just deleting "16*" in gc.c. Is there some sort of nasty quick fix like that I could do to just force it to work, or am I just going to have to pretend like every Julia process is going to eat 8GB of RAM? :P

Edit: I just found the 16* thing in the gc-pages.c, #define DEFAULT_REGION_PG_COUNT (16 * 8 * 4096) // 8 GB. I'm going to try to compile from source eliminating the "16*". Provided it works, I will reiterate the question asked under 10390 about why it's still the default if it seems to routinely cause problems in HPC environments. Would a PR be accepted that simply deleted it and made that always 512MB instead of 8GB?

[aviks]

This is marked fixed by #21135, but I did not see any user facing options in that PR. How is this meant to work, @vtjnash ?

[vtjnash]

That PR changes the GC to only allocate heap memory (RLIMIT_AS) as you use it. Thus the user-facing option is now simply not to allocate larger arrays than you intended to allocate.

[yuyichao]

The two are actually unrelated since the option is about heap usage not about pool size.

[yuyichao]

And FWIW https://github.com/JuliaLang/julia/issues/17987#issuecomment-285850134 is NOT what this issue is about.

[aviks]

Thanks for the explanation Jameson. That helps in many situations. But for this issue, the following is what I had in mind:

Say I own a machine with 8GB ram. I start a Julia process, and in that I run a program written by someone else. What I want to control is: if that program allocates anything more than 2Gb, throw an OutOfMemory error. Also, if the program has allocated 1.8GB, run the GC. How can I achieve that?

[joshjob42]

@yuyichao You said in #10390 that the issue remaining last September there (at the end of the thread) was no longer #10390 but was instead this #17987. Yet deleting the 16* as I described in my comment above (and was given as a "fix" in #10390) fixed the issue I (and others on that thread) was having. I came here with my comment only because of your comment on 10390 when you closed the issue. If #21135 resolves any/all problems potentially stemming from the 16* in gc-pages.c (by making it unnecessary/etc.), then it solves #10390 (or whatever lingering problems from there, which you identified as being this issue). I'm sorry if this issue isn't actually the one now covering the problem people were having in 10390; it wasn't intentional.

[vtjnash]

RLIMIT_AS can now be used to hard-limit max allocation size (although might need to add a bit of slack during configuration to account for openblas). Are we not already using it to configure the GC threshold also?

[yuyichao]

Are we not already using it to configure the GC threshold also?

No. It's also pretty wrong since the GC would have to guess how much memory other libraries uses. OpenBLAS might allocate a lot at start up but it's very far from the library that can allocate the most memory.

[vtjnash]

That would also roughly apply to the current usage of uv_get_total_memory

[yuyichao]

Kind of but not really. The current usage of total memory only set the max collection interval which is very different from total memory consumption.

[floswald]

just chipping in to say that I still face exactly the same problem as in my comment on #10390. I have julia showing up as consuming about 4GB of resident ram, but virtual memory required is more than twice that - hence kills my job on a 8GB compute node.