[libvirt RFC] virFile: new VIR_FILE_WRAPPER_BIG_PIPE to improve performance
Claudio Fontana
cfontana at suse.de
Mon Mar 14 17:38:31 UTC 2022
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
>>
>> 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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