[libvirt RFC] virFile: new VIR_FILE_WRAPPER_BIG_PIPE to improve performance

Claudio Fontana cfontana at suse.de
Mon Mar 28 13:28:38 UTC 2022


On 3/28/22 12:47 PM, Claudio Fontana wrote:
> On 3/26/22 4:49 PM, Claudio Fontana wrote:
>> On 3/25/22 12:29 PM, Daniel P. Berrangé wrote:
>>> On Fri, Mar 18, 2022 at 02:34:29PM +0100, Claudio Fontana wrote:
>>>> On 3/17/22 4:03 PM, Dr. David Alan Gilbert wrote:
>>>>> * Claudio Fontana (cfontana at suse.de) wrote:
>>>>>> On 3/17/22 2:41 PM, Claudio Fontana wrote:
>>>>>>> On 3/17/22 11:25 AM, Daniel P. Berrangé wrote:
>>>>>>>> On Thu, Mar 17, 2022 at 11:12:11AM +0100, Claudio Fontana wrote:
>>>>>>>>> On 3/16/22 1:17 PM, Claudio Fontana wrote:
>>>>>>>>>> On 3/14/22 6:48 PM, Daniel P. Berrangé wrote:
>>>>>>>>>>> On Mon, Mar 14, 2022 at 06:38:31PM +0100, Claudio Fontana wrote:
>>>>>>>>>>>> On 3/14/22 6:17 PM, Daniel P. Berrangé wrote:
>>>>>>>>>>>>> On Sat, Mar 12, 2022 at 05:30:01PM +0100, Claudio Fontana wrote:
>>>>>>>>>>>>>> the first user is the qemu driver,
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> virsh save/resume would slow to a crawl with a default pipe size (64k).
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> This improves the situation by 400%.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Going through io_helper still seems to incur in some penalty (~15%-ish)
>>>>>>>>>>>>>> compared with direct qemu migration to a nc socket to a file.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Signed-off-by: Claudio Fontana <cfontana at suse.de>
>>>>>>>>>>>>>> ---
>>>>>>>>>>>>>>  src/qemu/qemu_driver.c    |  6 +++---
>>>>>>>>>>>>>>  src/qemu/qemu_saveimage.c | 11 ++++++-----
>>>>>>>>>>>>>>  src/util/virfile.c        | 12 ++++++++++++
>>>>>>>>>>>>>>  src/util/virfile.h        |  1 +
>>>>>>>>>>>>>>  4 files changed, 22 insertions(+), 8 deletions(-)
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Hello, I initially thought this to be a qemu performance issue,
>>>>>>>>>>>>>> so you can find the discussion about this in qemu-devel:
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> "Re: bad virsh save /dev/null performance (600 MiB/s max)"
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> https://lists.gnu.org/archive/html/qemu-devel/2022-03/msg03142.html
>>>>>>>>
>>>>>>>>
>>>>>>>>> Current results show these experimental averages maximum throughput
>>>>>>>>> migrating to /dev/null per each FdWrapper Pipe Size (as per QEMU QMP
>>>>>>>>> "query-migrate", tests repeated 5 times for each).
>>>>>>>>> VM Size is 60G, most of the memory effectively touched before migration,
>>>>>>>>> through user application allocating and touching all memory with
>>>>>>>>> pseudorandom data.
>>>>>>>>>
>>>>>>>>> 64K:     5200 Mbps (current situation)
>>>>>>>>> 128K:    5800 Mbps
>>>>>>>>> 256K:   20900 Mbps
>>>>>>>>> 512K:   21600 Mbps
>>>>>>>>> 1M:     22800 Mbps
>>>>>>>>> 2M:     22800 Mbps
>>>>>>>>> 4M:     22400 Mbps
>>>>>>>>> 8M:     22500 Mbps
>>>>>>>>> 16M:    22800 Mbps
>>>>>>>>> 32M:    22900 Mbps
>>>>>>>>> 64M:    22900 Mbps
>>>>>>>>> 128M:   22800 Mbps
>>>>>>>>>
>>>>>>>>> This above is the throughput out of patched libvirt with multiple Pipe Sizes for the FDWrapper.
>>>>>>>>
>>>>>>>> Ok, its bouncing around with noise after 1 MB. So I'd suggest that
>>>>>>>> libvirt attempt to raise the pipe limit to 1 MB by default, but
>>>>>>>> not try to go higher.
>>>>>>>>
>>>>>>>>> As for the theoretical limit for the libvirt architecture,
>>>>>>>>> I ran a qemu migration directly issuing the appropriate QMP
>>>>>>>>> commands, setting the same migration parameters as per libvirt,
>>>>>>>>> and then migrating to a socket netcatted to /dev/null via
>>>>>>>>> {"execute": "migrate", "arguments": { "uri", "unix:///tmp/netcat.sock" } } :
>>>>>>>>>
>>>>>>>>> QMP:    37000 Mbps
>>>>>>>>
>>>>>>>>> So although the Pipe size improves things (in particular the
>>>>>>>>> large jump is for the 256K size, although 1M seems a very good value),
>>>>>>>>> there is still a second bottleneck in there somewhere that
>>>>>>>>> accounts for a loss of ~14200 Mbps in throughput.
>>>>>>
>>>>>>
>>>>>> Interesting addition: I tested quickly on a system with faster cpus and larger VM sizes, up to 200GB,
>>>>>> and the difference in throughput libvirt vs qemu is basically the same ~14500 Mbps.
>>>>>>
>>>>>> ~50000 mbps qemu to netcat socket to /dev/null
>>>>>> ~35500 mbps virsh save to /dev/null
>>>>>>
>>>>>> Seems it is not proportional to cpu speed by the looks of it (not a totally fair comparison because the VM sizes are different).
>>>>>
>>>>> It might be closer to RAM or cache bandwidth limited though; for an extra copy.
>>>>
>>>> I was thinking about sendfile(2) in iohelper, but that probably
>>>> can't work as the input fd is a socket, I am getting EINVAL.
>>>
>>> Yep, sendfile() requires the input to be a mmapable FD,
>>> and the output to be a socket.
>>>
>>> Try splice() instead  which merely requires 1 end to be a
>>> pipe, and the other end can be any FD afaik.
>>>
>>> With regards,
>>> Daniel
>>>
>>
>> I did try splice(), but performance is worse by around 500%.
>>
>> It also fails with EINVAL when trying to use it in combination with O_DIRECT.
>>
>> Tried larger and smaller buffers, flags like SPLICE_F_MORE an SPLICE_F_MOVE in any combination; no change, just awful performance.
> 
> 
> Ok I found a case where splice actually helps: in the read case, without O_DIRECT, splice seems to actually outperform read/write
> by _a lot_.


I was just hit by a cache effect. No real improvements I could measure.

> 
> I will code up the patch and start making more experiments with larger VM sizes etc.
> 
> Thanks!
> 
> Claudio
> 
> 
>>
>> Here is the code:
>>
>> #ifdef __linux__
>> +static ssize_t safesplice(int fdin, int fdout, size_t todo)
>> +{
>> +    unsigned int flags = SPLICE_F_MOVE | SPLICE_F_MORE;
>> +    ssize_t ncopied = 0;
>> +
>> +    while (todo > 0) {
>> +        ssize_t r = splice(fdin, NULL, fdout, NULL, todo, flags);
>> +        if (r < 0 && errno == EINTR)
>> +            continue;
>> +        if (r < 0)
>> +            return r;
>> +        if (r == 0)
>> +            return ncopied;
>> +        todo -= r;
>> +        ncopied += r;
>> +    }
>> +    return ncopied;
>> +}
>> +
>> +static ssize_t runIOCopy(const struct runIOParams p)
>> +{
>> +    size_t len = 1024 * 1024;
>> +    ssize_t total = 0;
>> +
>> +    while (1) {
>> +        ssize_t got = safesplice(p.fdin, p.fdout, len);
>> +        if (got < 0)
>> +            return -1;
>> +        if (got == 0)
>> +            break;
>> +
>> +        total += got;
>> +
>> +        /* handle last write truncate in direct case */
>> +        if (got < len && p.isDirect && p.isWrite && !p.isBlockDev) {
>> +            if (ftruncate(p.fdout, total) < 0) {
>> +                return -4;
>> +            }
>> +            break;
>> +        }
>> +    }
>> +    return total;
>> +}
>> +
>> +#endif
>>
>>
>> Any ideas welcome,
>>
>> Claudio
>>
> 



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