Re: [RFC] malloc: a question about arena_get2()

[Xishi Qiu <qiuxishi at huawei dot com>]

On 2015/12/3 15:50, Zhangjian (Bamvor) wrote:

> Hi,
> 

I tested the latest glibc, and it still have the problem.
There are ten transparent pages allocated.
cat /proc/xx/smaps |grep AnonHugePages

If I change "if (__builtin_expect (n <= narenas_limit - 1, 0))"
to "if (__builtin_expect (n < narenas_limit, 0))", it seems OK.
There is no transparent page allocated.

Thanks,
Xishi Qiu

> 
> On 2015/12/3 10:15, Xishi Qiu wrote:
>> Hi,
>>
>> In function arena_get2(), I'm not quite understand the comment
>> about the following code, why the underflow is OK?
> A little bit background, we found 200-300M memory consumption
> compare with our old system(glibc 2.11). After debugging, we
> found that each process we created got one THP instead of 4k
> page which is introduced by commit 41b81892f11f
> ("Handle ARENA_TEST correctly).
>> "if (__builtin_expect (n <= narenas_limit - 1, 0))"
> And this piece of code (glibc2.17) is as same as lastest glibc.
>>
>> I find v2.11 is "if (narenas < narenas_limit)".
>>
>> I do a simple test, and find the child thread will create a new
>> heap when call malloc(), and v2.11 is not.
>>
>> v2.17:
>> __libc_malloc()->arena_lock()->arena_get2()->_int_new_arena()->new_heap()
>> then the return addr of malloc() is aligned to 2M, this will lead
>> to alloc transparent page in kernel. So it consume more memory than v2.11
>> (v2.11 is only 4kb)even user only malloc but not write/read the area.
> Indeed, there is hundreds of threads in our system, it means that it takes
> hundreds of Mega bytes memory after malloc.
> 
> Regards
> 
> Bamvor
> 
>> v2.11:
>> public_mALLOc()->arena_lock()->arena_get2()->reused_arena()
>> and public_mALLOc() will finally call sYSMALLOc()->mmap() syscall.
>> so the return adddr is only aligned to 4kb.
>>
>>
>> Here is my test code. Please use "gcc test.c -lpthread" to
>> compile it.
>>
>> Thanks,
>> Xishi Qiu
>>
>> #include <stdio.h>
>> #include <stdlib.h>
>> #include <string.h>
>> #include <sys/types.h>
>> #include <sys/stat.h>
>> #include <unistd.h>
>> #include <pthread.h>
>>
>> void *run_thr_adp( void *p )
>> {
>> 	int size = 5000*1000;
>> 	char *test;
>>
>> 	printf("malloc start\n");
>> 	test = (char *)malloc(size);
>> 	if (test)
>> 		printf("malloc success, start=0x%lx, end=0x%lx\n",
>> 			test, test+size-1);
>> 	printf("malloc end\n");
>>
>> 	sleep(600);
>> 	return NULL;
>> }
>>
>> void startThread( void ( *thr )( void* ) )
>> {
>>     pthread_t pid;
>>     pthread_create( &pid, NULL, run_thr_adp, ( void* )thr );
>> }
>>
>> int main()
>> {
>> 	int i;
>> 	for (i = 0; i < 10; i++)
>> 	{
>> 		startThread(0);
>> //		sleep(1);
>> 	}
>> 	sleep(1000);
>>
>> 	return 0;
>> }
>>
> 
> 
> .
>