Re: [libvirt RFC] virFile: new VIR_FILE_WRAPPER_BIG_PIPE to improve performance

[Claudio Fontana <cfontana@suse.de>]

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@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@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.

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