Date: Wed, 14 Jun 2006 10:35:03 -0700 From: Jason Evans <jasone@FreeBSD.org> To: John Baldwin <jhb@freebsd.org> Cc: freebsd-current@freebsd.org, jasone@freebsd.org, Krassimir Slavchev <krassi@bulinfo.net> Subject: Re: memory leak in free() Message-ID: <449048C7.6090109@FreeBSD.org> In-Reply-To: <200606141023.51185.jhb@freebsd.org> References: <448FC3AF.9060606@bulinfo.net> <200606141023.51185.jhb@freebsd.org>
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John Baldwin wrote:
> On Wednesday 14 June 2006 04:07, Krassimir Slavchev wrote:
>
>>Hello,
>>
>>This simple code demonstrates the problem:
>>
>>int main ()
>>{
>> char* buffer1;
>> char* buffer2;
>> int size = 2*1024*1024 + 1;
>>
>>for(;;) {
>> buffer1 = (char *) malloc(size);
>> buffer2 = (char *) malloc(size);
>>
>> free(buffer1);
>> free(buffer2);
>> }
>>}
>>
>>The second free() does not free allocated memory if size >2Mb.
>>
>>On 6.1-STABLE all is OK.
>
>
> This is probably an issue with jemalloc, I've cc'd jasone@ who wrote the
> new malloc() in HEAD.
>
This is on a 32-bit system, right? If so, what's happening is that the
brk-managed space (data segment) is being fragmented, such that the
address space isn't returned to the OS. However, this is not really a
memory leak, since madvise() is called in order to let the kernel know
that the unused space need not be swapped out.
I was reluctant to allow allocations > 1MB to be carved from brk because
I knew this could happen, but people complained about it, so I added the
feature. In practice, I think the current implementation makes the
right tradeoff, but I have no strong feelings on the matter.
If this is causing you particular problems for some application, a
simple way to work around it is to decrease the data segment size for
the application. That will cause most/all allocations to be carved from
memory via mmap() instead.
Incidentally, this isn't an issue on 64-bit systems, since only mmap()
is used to request memory from the kernel.
Jason
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