On Fri, May 03, 2024 at 05:54:28PM -0300, Fabiano Rosas wrote: > Peter Xu <pet...@redhat.com> writes: > > > On Fri, Apr 26, 2024 at 11:20:38AM -0300, Fabiano Rosas wrote: > >> When multifd is used along with mapped-ram, we can take benefit of a > >> filesystem that supports the O_DIRECT flag and perform direct I/O in > >> the multifd threads. This brings a significant performance improvement > >> because direct-io writes bypass the page cache which would otherwise > >> be thrashed by the multifd data which is unlikely to be needed again > >> in a short period of time. > >> > >> To be able to use a multifd channel opened with O_DIRECT, we must > >> ensure that a certain aligment is used. Filesystems usually require a > >> block-size alignment for direct I/O. The way to achieve this is by > >> enabling the mapped-ram feature, which already aligns its I/O properly > >> (see MAPPED_RAM_FILE_OFFSET_ALIGNMENT at ram.c). > >> > >> By setting O_DIRECT on the multifd channels, all writes to the same > >> file descriptor need to be aligned as well, even the ones that come > >> from outside multifd, such as the QEMUFile I/O from the main migration > >> code. This makes it impossible to use the same file descriptor for the > >> QEMUFile and for the multifd channels. The various flags and metadata > >> written by the main migration code will always be unaligned by virtue > >> of their small size. To workaround this issue, we'll require a second > >> file descriptor to be used exclusively for direct I/O. > >> > >> The second file descriptor can be obtained by QEMU by re-opening the > >> migration file (already possible), or by being provided by the user or > >> management application (support to be added in future patches). > >> > >> Signed-off-by: Fabiano Rosas <faro...@suse.de> > >> --- > >> migration/file.c | 22 +++++++++++++++++++--- > >> migration/migration.c | 23 +++++++++++++++++++++++ > >> 2 files changed, 42 insertions(+), 3 deletions(-) > >> > >> diff --git a/migration/file.c b/migration/file.c > >> index 8f30999400..b9265b14dd 100644 > >> --- a/migration/file.c > >> +++ b/migration/file.c > >> @@ -83,17 +83,33 @@ void file_cleanup_outgoing_migration(void) > >> > >> bool file_send_channel_create(gpointer opaque, Error **errp) > >> { > >> - QIOChannelFile *ioc; > >> + QIOChannelFile *ioc = NULL; > >> int flags = O_WRONLY; > >> - bool ret = true; > >> + bool ret = false; > >> + > >> + if (migrate_direct_io()) { > >> +#ifdef O_DIRECT > >> + /* > >> + * Enable O_DIRECT for the secondary channels. These are used > >> + * for sending ram pages and writes should be guaranteed to be > >> + * aligned to at least page size. > >> + */ > >> + flags |= O_DIRECT; > >> +#else > >> + error_setg(errp, "System does not support O_DIRECT"); > >> + error_append_hint(errp, > >> + "Try disabling direct-io migration > >> capability\n"); > >> + goto out; > >> +#endif > > > > Hopefully if we can fail migrate-set-parameters correctly always, we will > > never trigger this error. > > > > I know Linux used some trick like this to even avoid such ifdefs: > > > > if (qemu_has_direct_io() && migrate_direct_io()) { > > // reference O_DIRECT > > } > > > > So as long as qemu_has_direct_io() can return a constant "false" when > > O_DIRECT not defined, the compiler is smart enough to ignore the O_DIRECT > > inside the block. > > > > Even if it won't work, we can still avoid that error (and rely on the > > set-parameter failure): > > > > #ifdef O_DIRECT > > if (migrate_direct_io()) { > > // reference O_DIRECT > > } > > #endif > > > > Then it should run the same, just to try making ifdefs as light as > > possible.. > > Ok. > > Just FYI, in v2 I'm adding direct-io to migration incoming side as well, > so I put this logic into a helper: > > static bool file_enable_direct_io(int *flags, Error **errp) > { > if (migrate_direct_io()) { > #ifdef O_DIRECT > *flags |= O_DIRECT; > #else > error_setg(errp, "System does not support O_DIRECT"); > error_append_hint(errp, > "Try disabling direct-io migration capability\n"); > return false; > #endif > } > > return true; > } > > But I'll apply your suggestions nonetheless.
Thanks, please give it a shot, I hope it will work with either way. One thing to mention is, if you want to play with the qemu_has_direct_io() approach with no "#ifdefs", you can't keep qemu_has_direct_io() in osdep.c, but you must define it in osdep.h as static inline functions. Otherwise I think osdep.o is forced to include it as a function so that trick won't work. Just try compile without O_DIRECT should see. -- Peter Xu