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. > >> + } >> >> ioc = qio_channel_file_new_path(outgoing_args.fname, flags, 0, errp); >> if (!ioc) { >> - ret = false; >> goto out; >> } >> >> multifd_channel_connect(opaque, QIO_CHANNEL(ioc)); >> + ret = true; >> >> out: >> /* >> diff --git a/migration/migration.c b/migration/migration.c >> index b5af6b5105..cb923a3f62 100644 >> --- a/migration/migration.c >> +++ b/migration/migration.c >> @@ -155,6 +155,16 @@ static bool migration_needs_seekable_channel(void) >> return migrate_mapped_ram(); >> } >> >> +static bool migration_needs_multiple_fds(void) > > If I suggest to rename this, would you agree? :) > Sure, although this is the more accurate usage than "multifd" hehe. > I'd try with "migrate_needs_extra_fd()" or "migrate_needs_two_fds()", > or... just to avoid "multi" + "fd" used altogether, perhaps. > > Other than that looks all good. > > Thanks, > >> +{ >> + /* >> + * When doing direct-io, multifd requires two different, >> + * non-duplicated file descriptors so we can use one of them for >> + * unaligned IO. >> + */ >> + return migrate_multifd() && migrate_direct_io(); >> +} >> + >> static bool transport_supports_seeking(MigrationAddress *addr) >> { >> if (addr->transport == MIGRATION_ADDRESS_TYPE_FILE) { >> @@ -164,6 +174,12 @@ static bool transport_supports_seeking(MigrationAddress >> *addr) >> return false; >> } >> >> +static bool transport_supports_multiple_fds(MigrationAddress *addr) >> +{ >> + /* file: works because QEMU can open it multiple times */ >> + return addr->transport == MIGRATION_ADDRESS_TYPE_FILE; >> +} >> + >> static bool >> migration_channels_and_transport_compatible(MigrationAddress *addr, >> Error **errp) >> @@ -180,6 +196,13 @@ >> migration_channels_and_transport_compatible(MigrationAddress *addr, >> return false; >> } >> >> + if (migration_needs_multiple_fds() && >> + !transport_supports_multiple_fds(addr)) { >> + error_setg(errp, >> + "Migration requires a transport that allows for multiple >> fds (e.g. file)"); >> + return false; >> + } >> + >> return true; >> } >> >> -- >> 2.35.3 >>