On Monday, March 30, 2020 at 4:33:05 AM UTC-4, Nadav Har'El wrote:
>
> On Mon, Mar 30, 2020 at 7:38 AM Waldek Kozaczuk <[email protected]
> <javascript:>> wrote:
>
>> Hi,
>>
>> I came up with this test trying to replicate the fragmentation scenario
>> Rick Payne was experiencing:
>>
>> int main() {
>> size_t size = 0x4C1000;
>>
>> void* mem = malloc(size);
>> auto start = std::chrono::high_resolution_clock::now();
>>
>> for (int i = 0; i < 100; i++) {
>> size = (size * 105) / 100;
>> malloc(0x4000);
>> printf("%d allocation of %ld bytes\n", i + 1, size);
>> mem = realloc(mem, size);
>> }
>>
>> auto end = std::chrono::high_resolution_clock::now();
>> std::chrono::duration<double> elapsed = end - start;
>> std::cout << "Elapsed time: " << elapsed.count() << " s\n";
>> printf("DONE - last size: %ld!\n", size);
>>
>> sleep(1000);
>> free(mem);
>> }
>>
>> So before the latest patch that implements mapped-based malloc_large()
>> this test needs at least 1.6GB of memory to pass - the last realloc() would
>> try to allocate ~ 625MB or memory. However, I did not really see the
>> fragmentation I expected to see in physical memory so I do not think this
>> test is very representative.
>>
>> The latest version of the code with all patches applied would need a
>> minimum of 1.2GB - which is not surprising given that realloc() needs two
>> copies of similar size to copy data from one place to another one until it
>> frees the smaller one. But the good thing is, it does not need contiguous
>> 625MB of physical memory to do it anymore.
>>
>> However, with this experiment, I noticed that new malloc_large is slower
>> - this test takes 3 seconds vs 2 seconds before the patch. As I suspected
>> the culprit was the fact that mapped_malloc_large calls map_anon with
>> mmap_populate that pre-faults entire memory which I thought was necessary
>> to do. But it turns out that when I replaced mmap_populate
>> with mmap_uninitialized all the unit test and a couple of other apps were
>> still working just fine. And the test above would take a similar amount of
>> time as before - around 2 seconds. So maybe we should use
>> mmap_uninitialized. The only concern would be kernel code running in
>> preemption disabled mode and trying to access memory that was allocated
>> with mapped_malloc_large(). Could it happen?
>>
>
> Although malloc() cannot be called in preemption disabled mode (it uses
> locks), you're right that all preemption-disabled kernel code that *uses*
> memory previously allocated today just silently assumes that this memory is
> already populated ("swapped in"). There is even more delicate issues like
> the issue of lazy TLB flush explained in commit
> c9e5af6deef6ef9420bdf6437f8c34371c8df4e9. So this sort of
> preemption-disabled kernel code today relies on the old-style pre-populated
> and visible-on-all-cores malloc(), and may not work correctly with mmap().
> However, I don't think this sort of preemption-disabled code in the kernel
> ever works with very large allocations for which you fall back to mmap()?
>
With my patches malloc_large() uses map_anon() for any allocations >= 2MB
(greater than so called "huge pages) and for allocations > 4K and < 2MB
ONLY if we cannot find large enough page ranges that would fit
("fallback"). So do you think it is safe to change mmap_populate to
mmap_uninitialized for the 1st case but not for the other? It is based on
your thinking that kernel does not operate on such large allocated memory
areas?
Also as I understand each CPU has it own TLB so it has to be flushed to see
any changes made to the global page tables structures, right? So the commit
("mmu: flush tlb lazily for cpus that are running system threads" -
https://github.com/cloudius-systems/osv/commit/7e38453390d6c0164a72e30b2616b0f3c3025349
by
Gleb) optimizes flushing logic by making it lazy for system (?) threads
(reading the code in this commit I think we are optimizing the application
threads). This somehow affected the scheduler code which would sometimes
end up operating on mmapped areas of memory where thread info used to be
stored and lead to the a page fault when preemption was disabled, right?.
So your commit ("sched: only allocate sched::thread objects on the heap" -
https://github.com/cloudius-systems/osv/commit/c9e5af6deef6ef9420bdf6437f8c34371c8df4e9)
addressed it by making sure "new thread" is hidden by thread::make() that
makes sure to use regular non-mmaped way of allocating memory. So we hope
that my commit did not break any of that. Also even if we change to lazy
population for >2GB allocation we should be fine as the allocation made by
thread::make() should not try to allocate more than 4K of memory and call
malloc_large(), right?
static thread* make(Args&&... args) {
return new thread(std::forward<Args>(args)...);
}
Lastly, do you think the changes to malloc_large() affect the patch I sent
to make application threads use lazily populated stack
- https://groups.google.com/d/msg/osv-dev/tZnwiScmjZY/GkY0hV9EAwAJ ?
>> Finally, when I ran the same test on Linux host it would complete under
>> 2ms (milliseconds) - so 1000 times faster. Why is that? Linux obviously
>> uses mremap() to implement reallloc() and OSv copies the data using
>> memcpy(). So if we want to make realloc() work faster (it could be also
>> useful to speedup ramfs), we should implement remap() - here is an open
>> issue for that - https://github.com/cloudius-systems/osv/issues/184 and
>> supposedly Pekka sent a patch a while ago which we can build upon.
>>
>
> As I noted a few weeks ago on this mailing list, Linux had hundreds of
> people optimizing every tiny detail. The question is whether this
> optimization is worth it. Is it common for people to realloc() huge
> allocations? If they do, do they do it many times, or exponentially (i.e.,
> double the size each time)?
> If you think it's worth it, then, yes - your new malloc() code now knows
> it used mmap(), and can use mremap() to do the actual work. I think the
> question is just if you want to optimize this specific use case.
>
>
>>
>> Waldek
>>
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>>
>
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