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

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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@xxxxxxx>
>> ---
>>  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
>>
>> RFC since need to validate idea, and it is only lightly tested:
>>
>> save     - about 400% benefit in throughput, getting around 20 Gbps to /dev/null,
>>            and around 13 Gbps to a ramdisk.
>> 	   By comparison, direct qemu migration to a nc socket is around 24Gbps.
>>
>> restore  - not tested, _should_ also benefit in the "bypass_cache" case
>> coredump - not tested, _should_ also benefit like for save
>>
>> Thanks for your comments and review,
>>
>> Claudio
>>
>>
>> diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
>> index c1b3bd8536..be248c1e92 100644
>> --- a/src/qemu/qemu_driver.c
>> +++ b/src/qemu/qemu_driver.c
>> @@ -3044,7 +3044,7 @@ doCoreDump(virQEMUDriver *driver,
>>      virFileWrapperFd *wrapperFd = NULL;
>>      int directFlag = 0;
>>      bool needUnlink = false;
>> -    unsigned int flags = VIR_FILE_WRAPPER_NON_BLOCKING;
>> +    unsigned int wrapperFlags = VIR_FILE_WRAPPER_NON_BLOCKING | VIR_FILE_WRAPPER_BIG_PIPE;
>>      const char *memory_dump_format = NULL;
>>      g_autoptr(virQEMUDriverConfig) cfg = virQEMUDriverGetConfig(driver);
>>      g_autoptr(virCommand) compressor = NULL;
>> @@ -3059,7 +3059,7 @@ doCoreDump(virQEMUDriver *driver,
>>  
>>      /* Create an empty file with appropriate ownership.  */
>>      if (dump_flags & VIR_DUMP_BYPASS_CACHE) {
>> -        flags |= VIR_FILE_WRAPPER_BYPASS_CACHE;
>> +        wrapperFlags |= VIR_FILE_WRAPPER_BYPASS_CACHE;
>>          directFlag = virFileDirectFdFlag();
>>          if (directFlag < 0) {
>>              virReportError(VIR_ERR_OPERATION_FAILED, "%s",
>> @@ -3072,7 +3072,7 @@ doCoreDump(virQEMUDriver *driver,
>>                               &needUnlink)) < 0)
>>          goto cleanup;
>>  
>> -    if (!(wrapperFd = virFileWrapperFdNew(&fd, path, flags)))
>> +    if (!(wrapperFd = virFileWrapperFdNew(&fd, path, wrapperFlags)))
>>          goto cleanup;
>>  
>>      if (dump_flags & VIR_DUMP_MEMORY_ONLY) {
>> diff --git a/src/qemu/qemu_saveimage.c b/src/qemu/qemu_saveimage.c
>> index c0139041eb..1b522a1542 100644
>> --- a/src/qemu/qemu_saveimage.c
>> +++ b/src/qemu/qemu_saveimage.c
>> @@ -267,7 +267,7 @@ qemuSaveImageCreate(virQEMUDriver *driver,
>>      int fd = -1;
>>      int directFlag = 0;
>>      virFileWrapperFd *wrapperFd = NULL;
>> -    unsigned int wrapperFlags = VIR_FILE_WRAPPER_NON_BLOCKING;
>> +    unsigned int wrapperFlags = VIR_FILE_WRAPPER_NON_BLOCKING | VIR_FILE_WRAPPER_BIG_PIPE;
>>  
>>      /* Obtain the file handle.  */
>>      if ((flags & VIR_DOMAIN_SAVE_BYPASS_CACHE)) {
>> @@ -463,10 +463,11 @@ qemuSaveImageOpen(virQEMUDriver *driver,
>>      if ((fd = qemuDomainOpenFile(cfg, NULL, path, oflags, NULL)) < 0)
>>          return -1;
>>  
>> -    if (bypass_cache &&
>> -        !(*wrapperFd = virFileWrapperFdNew(&fd, path,
>> -                                           VIR_FILE_WRAPPER_BYPASS_CACHE)))
>> -        return -1;
>> +    if (bypass_cache) {
>> +        unsigned int wrapperFlags = VIR_FILE_WRAPPER_BYPASS_CACHE | VIR_FILE_WRAPPER_BIG_PIPE;
>> +        if (!(*wrapperFd = virFileWrapperFdNew(&fd, path, wrapperFlags)))
>> +            return -1;
>> +    }
>>  
>>      data = g_new0(virQEMUSaveData, 1);
>>  
>> diff --git a/src/util/virfile.c b/src/util/virfile.c
>> index a04f888e06..fdacd17890 100644
>> --- a/src/util/virfile.c
>> +++ b/src/util/virfile.c
>> @@ -282,6 +282,18 @@ virFileWrapperFdNew(int *fd, const char *name, unsigned int flags)
>>  
>>      ret->cmd = virCommandNewArgList(iohelper_path, name, NULL);
>>  
>> +    if (flags & VIR_FILE_WRAPPER_BIG_PIPE) {
>> +        /*
>> +         * virsh save/resume would slow to a crawl with a default pipe size (usually 64k).
>> +         * This improves the situation by 400%, although going through io_helper still incurs
>> +         * in a performance penalty compared with a direct qemu migration to a socket.
>> +         */
>> +        int pipe_sz, rv = virFileReadValueInt(&pipe_sz, "/proc/sys/fs/pipe-max-size");
> 
> This is fine as an experiment but I don't think it is that safe
> to use in the real world. There could be a variety of reasons why
> an admin can enlarge this value, and we shouldn't assume the max
> size is sensible for libvirt/QEMU to use.
> 
> I very much suspect there are diminishing returns here in terms
> of buffer sizes.
> 
> 64k is obvious too small, but 1 MB, may be sufficiently large
> that the bottleneck is then elsewhere in our code. IOW, If the
> pipe max size is 100 MB, we shouldn't blindly use it. Can you
> do a few tests with varying sizes to see where a sensible
> tradeoff falls ?


Hi Daniel,

this is a very good point. Actually I see very diminishing returns after the default pipe-max-size (1MB).

The idea was that beyond allowing larger size, the admin could have set a _smaller_ pipe-max-size,
so we want to use that in that case, otherwise an attempt to use 1MB would result in EPERM, if the process does not have CAP_SYS_RESOURCE or CAP_SYS_ADMIN.
I am not sure if used with Kubevirt, for example, CAP_SYS_RESOURCE or CAP_SYS_ADMIN would be available...?

So maybe one idea could be to use the minimum between /proc/sys/fs/pipe-max-size and for example 1MB, but will do more testing to see where the actual break point is.

Wdyt?

Thanks!

Claudio


> 
>> +        if (rv != 0) {
>> +            pipe_sz = 1024 * 1024; /* common default for pipe-max-size */
>> +        }
>> +        fcntl(pipefd[output ? 0 : 1], F_SETPIPE_SZ, pipe_sz);
>> +    }
> 
> Regards,
> Daniel
> 





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