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[RFC] malloc: a question about arena_get2()


In function arena_get2(), I'm not quite understand the comment
about the following code, why the underflow is OK?
"if (__builtin_expect (n <= narenas_limit - 1, 0))"

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.

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.

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.

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");

	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++)
//		sleep(1);

	return 0;

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