[libvirt RFCv3] virfile: set pipe size in virFileWrapperFdNew to improve throughput

[Claudio Fontana]

On 3/25/22 2:54 PM, Daniel P. Berrangé wrote:
> On Fri, Mar 25, 2022 at 02:52:05PM +0100, Claudio Fontana wrote:
>> On 3/25/22 2:13 PM, Daniel P. Berrangé wrote:
>>> On Fri, Mar 25, 2022 at 01:54:51PM +0100, Claudio Fontana wrote:
>>>> currently the only user of virFileWrapperFdNew is the qemu driver;
>>>> virsh save is very slow with a default pipe size.
>>>> This change improves throughput by ~400% on fast nvme or ramdisk.
>>>>
>>>> Best value currently measured is 1MB, which happens to be also
>>>> the kernel default for the pipe-max-size.
>>>>
>>>> Signed-off-by: Claudio Fontana <cfontana at suse.de>
>>>> ---
>>>>
>>>> see v2 at
>>>> https://listman.redhat.com/archives/libvir-list/2022-March/229423.html
>>>>
>>>> Changes v2 -> v3:
>>>>
>>>> * removed reading of max-pipe-size from procfs,
>>>>   instead make multiple attempts on EPERM with smaller sizes.
>>>>   In the regular case, this should succeed on the first try.
>>>>   (Daniel)
>>>>
>>>> Changes v1 -> v2:
>>>>
>>>> * removed VIR_FILE_WRAPPER_BIG_PIPE, made the new pipe resizing
>>>>   unconditional (Michal)
>>>>
>>>> * moved code to separate functions (Michal)
>>>>
>>>> * removed ternary op, disliked in libvirt (Michal)
>>>>
>>>> * added #ifdef __linux__ (Ani Sinha)
>>>>
>>>> * try smallest value between currently best measured value (1MB)
>>>>   and the pipe-max-size setting. If pipe-max-size cannot be read,
>>>>   try kernel default max (1MB). (Daniel)
>>>>
>>>>
>>>> src/util/virfile.c | 49 ++++++++++++++++++++++++++++++++++++++++++++++
>>>>  1 file changed, 49 insertions(+)
>>>>
>>>> diff --git a/src/util/virfile.c b/src/util/virfile.c
>>>> index a04f888e06..876b865974 100644
>>>> --- a/src/util/virfile.c
>>>> +++ b/src/util/virfile.c
>>>> @@ -201,6 +201,51 @@ struct _virFileWrapperFd {
>>>>  };
>>>>  
>>>>  #ifndef WIN32
>>>> +
>>>> +#ifdef __linux__
>>>> +
>>>> +/**
>>>> + * virFileWrapperSetPipeSize:
>>>> + * @fd: the fd of the pipe
>>>> + *
>>>> + * Set best pipe size on the passed file descriptor for bulk transfers of data.
>>>> + *
>>>> + * default pipe size (usually 64K) is generally not suited for large transfers
>>>> + * to fast devices. A value of 1MB has been measured to improve virsh save
>>>> + * by 400% in ideal conditions. We retry multiple times with smaller sizes
>>>> + * on EPERM to account for possible small values of /proc/sys/fs/pipe-max-size.
>>>> + *
>>>> + * Return value is 0 on success, -1 and errno set on error.
>>>> + * OS note: only for linux, on other OS this is a no-op.
>>>> + */
>>>> +static int
>>>> +virFileWrapperSetPipeSize(int fd)
>>>> +{
>>>> +    int sz;
>>>> +
>>>> +    for (sz = 1024 * 1024; sz >= 64 * 1024; sz /= 2) {
>>>> +        int rv = fcntl(fd, F_SETPIPE_SZ, sz);
>>>> +        if (rv < 0 && errno == EPERM) {
>>>> +            continue; /* retry with half the size */
>>>> +        }
>>>> +        if (rv < 0) {
>>>> +            break;
>>>> +        }
>>>> +        VIR_INFO("fd %d pipe size adjusted to %d", fd, sz);
>>>> +        return 0;
>>>> +    }
>>>> +    VIR_WARN("failed to set pipe size to %d (errno=%d)", sz, errno);
>>>> +    return -1;
>>>> +}
>>>> +
>>>> +#else /* !__linux__ */
>>>> +static int virFileWrapperSetPipeSize(int fd)
>>>> +{
>>>> +    return 0;
>>>> +}
>>>> +#endif /* !__linux__ */
>>>> +
>>>> +
>>>>  /**
>>>>   * virFileWrapperFdNew:
>>>>   * @fd: pointer to fd to wrap
>>>> @@ -282,6 +327,10 @@ virFileWrapperFdNew(int *fd, const char *name, unsigned int flags)
>>>>  
>>>>      ret->cmd = virCommandNewArgList(iohelper_path, name, NULL);
>>>>  
>>>> +    if (virFileWrapperSetPipeSize(pipefd[!output]) < 0) {
>>>> +        virReportError(VIR_ERR_SYSTEM_ERROR, "%s", _("unable to set pipe size, data transfer might be slow"));
>>>
>>> Push this into virFileWrapperSetPipeSize instead of the VIR_WARN
>>> there, and use virReportSystemError passing in the errno value too.
>>
>>
>> ok, what about also warning on EPERM? In the normal case we should succeed on the first try I think.
> 
> We generally try to avoid any VIR_WARN in cases that we expect to be
> still functional. Users tend to complain when they get warnings for

INFO? DEBUG? Or nothing at all? Thanks again

> these kind of things. I think coping with smaller max size is a normal
> situation, so its merely a perf factor, not a functional problem.
> 
> With regards,
> Daniel
>