Re: [dpdk-dev] 答复: [PATCH v2] mem: accelerate dpdk program startup by reuse page from page cache

["Burakov, Anatoly" <anatoly.burakov@intel.com>]

On 11-Nov-18 2:19 AM, 范建明 wrote:
> Hi, Burakov
> 
>    Thanks very much for your reply.
>   
>    I run the testpmd dpdk18.08 on my router with 200GB huge page configured.
>    And find it still takes 50s in zeroing the 200GB huge page each time the program restarts.
>    
> 
>    As you mentioned app shall calls rte_eal_cleanup() to do the cleanup.
>    However, this api is not designed to accelerate startup time.
>    >>
>       During rte_eal_init() EAL allocates memory from hugepages to enable its core libraries to perform their tasks.
> 	 The rte_eal_cleanup() function releases these resources, ensuring that no hugepage memory is leaked.
> 	 It is expected that all DPDK applications call rte_eal_cleanup() before exiting.
> 	 Not calling this function could result in leaking hugepages, leading to failure during initialization of secondary processes.
>    >>
>    I guess you suggest to use secondary process which uses share memory for fast startup. However, as you know, exist applications need to do a lot of change to use it well.

Hi,

No, what i was suggesting is to use this function on application exit 
(as well as free any other used memory, such as mempools, hashtables, 
minor things allocated through rte_malloc, etc.). This will allow the 
memory subsystem to release hugepage memory back to the system after 
DPDK shutdown.

>    
>   And You mentioned faster initialization is one of the key reasons why the new memory subsystem was developed.
>   However, as I read the following code, I guess the community doesn't really consider the power of reuse the exist hugepage fs page cache.
> 	hugepage_info_init(void)
> 	{
>     		/	* clear out the hugepages dir from unused pages */
>     		if (clear_hugedir(hpi->hugedir) == -1)
>       		 break;
> 	}
> 

...which wouldn't be necessary if those hugepages weren't used in the 
first place! If your DPDK process has released all of its memory before 
shutdown, there would have been no hugepages to reuse. Granted, our 
sample applications do not follow this convention and routinely leak 
memory, but it is possible to fix this problem.

For example, if you run test application from current master without any 
hardware devices, and then exit - there will be no leftover hugepage 
files after test app exit. That's because test application will clean 
after itself and release all hugepage memory back to the system at 
shutdown. For example:

anatoly@xxx:~$ ls /mnt/huge*/
/mnt/huge/:

/mnt/huge_1G/:

### as you can see, no hugepages are allocated
### now run test app with non-existent hardware

anatoly@xxx:~$ sudo build/DPDK/app/test -w 01:00.0
EAL: Detected 88 lcore(s)
EAL: Detected 2 NUMA nodes
EAL: Multi-process socket /var/run/dpdk/rte/mp_socket
EAL: Probing VFIO support...
EAL: VFIO support initialized
EAL: PCI device 0000:01:00.0 on NUMA socket 0
EAL:   probe driver: 8086:1521 net_e1000_igb
APP: HPET is not enabled, using TSC as default timer
RTE>>memzone_autotest

### runs memzone autotest, which allocates and free a bunch of memory

RTE>>quit

### test app quit, now check hugetlbfs again

anatoly@silpixa00399498:~$ ls /mnt/huge*/
/mnt/huge/:

/mnt/huge_1G/:

### as you can see - no hugepages are leftover from execution

The "non-existent device" part is because currently, test application 
does not clean up any driver-allocated memory (and i don't even think 
there's a way to do that without hot-unplugging everything right before 
shutdown), but the key point is - you wouldn't have had gigabytes' worth 
of hugepages to clean if you cleaned up all your memory before 
application shutdown. Device memory takes up maybe 10-12 megabytes per 
device (depending on device, of course).

Also, like Stephen mentioned, leftover hugepages would still be a 
problem after a crash, but you'd get *huge* problems reusing that 
memory, which is great reason to dislike this patch as well. But even 
putting crash scenarios aside, given that in normal and correct API 
usage, the problem you are trying to solve is not a problem in the first 
place, and i don't think reusing page cache is the correct way to do 
this. The proper way to fix the problem is to fix your application to 
release all of its DPDK memory on shutdown.

> 
> 
>    The key to this patch is that it takes advantage of the page cache of huge page fs.
>    with this patch, when you first startup the program, the following steps by be taken.
>    
>    1. user space: create files under /dev/hugepages
>    2. user space: do mmap with shared and populate flag set.
>    3. kernel space:
>        3.1 find free vma.
> 	  3.2 alloc huge page from huge page pool reserved by the hugepage fs.
> 	  3.3 call clear_huge_page to zero the page.
> 			******************This step is very time-consuming********************
> 		
> 	  3.4 insert the page to the file inode's page cache
> 	  3.5 insert the page into the page table
> 	
> 	
>    then if you restart the program, the following steps will be taken
>    1. user space: open files under /dev/hugepages.
>    2. user space: do mmap with shared and populate flag set.
>    3. kernel space:
>        3.1 find free vma
> 	  3.2 it search the file's inode page cache, and find there is page there.
> 	  3.3 insert the page into the page table
>    Note restart the program doesn't need to do clear_huge_page any more!!!
>    
> 
>    Btw, i worked for intel serveral years ago. It's a great place to work.
> 
> Best regards
> jianming
> 
> -----邮件原件-----
> 发件人: Burakov, Anatoly [mailto:anatoly.burakov@intel.com]
> 发送时间: 2018年11月9日 22:03
> 收件人: jianmingfan <jianmingfan@126.com>; dev@dpdk.org
> 抄送: 范建明 <fanjianming@jd.com>
> 主题: Re: [dpdk-dev] [PATCH v2] mem: accelerate dpdk program startup by reuse page from page cache
> 
> On 09-Nov-18 12:20 PM, Burakov, Anatoly wrote:
>> On 09-Nov-18 9:23 AM, jianmingfan wrote:
>>> --- fix coding style of the previous patch
>>>
>>> During procless startup, dpdk invokes clear_hugedir() to unlink all
>>> hugepage files under /dev/hugepages. Then in map_all_hugepages(), it
>>> invokes mmap to allocate and zero all the huge pages as configured in
>>> /sys/kernel/mm/hugepages/xxx/nr_hugepages.
>>>
>>> This cause startup process extreamly slow with large size of huge
>>> page configured.
>>>
>>> In our use case, we usually configure as large as 200GB hugepages in
>>> our router. It takes more than 50s each time dpdk process startup to
>>> clear the pages.
>>>
>>> To address this issue, user can turn on --reuse-map switch. With it,
>>> dpdk will check the validity of the exiting page cache under
>>> /dev/hugespages. If valid, the cache will be reused not deleted, so
>>> that the os doesn't need to zero the pages again.
>>>
>>> However, as there are a lot of users ,e.g. rte_kni_alloc, rely on the
>>> os zeor page behavior. To keep things work, I add memset during
>>> malloc_heap_alloc(). This makes sense due to the following reason.
>>> 1) user often configure hugepage size too large to be used by the
>>> program.
>>> In our router, 200GB is configured, but less than 2GB is actually used.
>>> 2) dpdk users don't call heap allocation in performance-critical path.
>>> They alloc memory during process bootup.
>>>
>>> Signed-off-by: Jianming Fan <fanjianming@jd.com>
>>> ---
>>
>> I believe this issue is better solved by actually fixing all of the
>> memory that DPDK leaves behind. We already have rte_eal_cleanup() call
>> which will deallocate any EAL-allocated memory that have been
>> reserved, and an exited application should free any memory it was
>> using so that memory subsystem could free it back to the system,
>> thereby not needing any cleaning of hugepages at startup.
>>
>> If your application does not e.g. free its mempools on exit, it should
>> :) Chances are, the problem will go away. The only circumstance where
>> this may not work is if you preallocated your memory using
>> -m/--socket-mem flag.
>>
> 
> To clarify - all of the above is only applicable to 18.05 and beyond.
> The map_all_hugepages() function only gets called in the legacy mem
> init, so this patch solves a problem that does not exist on recent DPDK
> versions in the first place - faster initialization is one of the key
> reasons why the new memory subsystem was developed.
> 


-- 
Thanks,
Anatoly