On Mon, Mar 14, 2022 at 5:06 PM Michal Prívozník <mprivozn@xxxxxxxxxx> wrote: > > On 3/12/22 17:30, 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 > > Hey, I like this idea, but couple of points below. > > > > > > > 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) { > > > I believe we don't need this flag. I mean, the plain fact that > virFileWrapper is used means that caller wants to avoid VFS because it's > interested in speed. Therefore, this code could be done unconditionally. > > > + /* > > + * 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. > > + */ > > This belongs into the commit message. This code has no knowledge about > qemu. What you can mention here is the performance benefit. > Also, QEMU migrating straight to a socket is going to have performance > benefit but only in a few cases, because if it's a migration into a file > then VFS (and thus caching) is involved. Thus, if you migrate into a > file and have enough free RAM for caches then yes, it's going to be > faster. But if you don't have free RAM then it's going to be way slower. > > > + int pipe_sz, rv = virFileReadValueInt(&pipe_sz, "/proc/sys/fs/pipe-max-size"); > > + if (rv != 0) { > > + pipe_sz = 1024 * 1024; /* common default for pipe-max-size */ > > + } > > + fcntl(pipefd[output ? 0 : 1], F_SETPIPE_SZ, pipe_sz); > > Alternative implementation would be to call fcntl() only if we know > we've succeeded in reading /proc/.../pipe-max-size, like this: > > int pipe_sz; > int rv = virFileReadValueInt(&pipe_size, "/proc/sys/fs/pipe-max-size"); > > if (rv >= 0) > fcntl(pipefd[output ? 0 : 1], F_SETPIPE_SZ, pipe_sz); > > (notice I've declared variables on separate lines, we like it that way) > > Now, what can we do about that ternary operator? It doesn't look nice. > But I guess we can hardcode just one end of the pipe, because it's the > actual pipe and not FD we are modifying here. > > Lastly, let's add some error checking: > > int pipe_sz; > int rv = virFileReadValueInt(&pipe_size, "/proc/sys/fs/pipe-max-size"); > > if (rv == -2) { > /* file doesn't exist */ Yes here we need to distinguish between error on linux vs legit error on non-linux systems.
