Concurrent small allocation defeats large allocation #8843
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FYI, this is a port of openzfs/openzfs#732 |
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@pcd1193182 it looks like CI didn't get notified of this PR. Would you mind force updating the PR to force it to run. |
behlendorf
requested changes
Jun 7, 2019
behlendorf
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Status: Revision Needed
Signed-off-by: Paul Dagnelie <[email protected]> External-issue: DLPX-61314
Codecov Report
@@ Coverage Diff @@
## master #8843 +/- ##
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+ Coverage 78.48% 78.62% +0.13%
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Files 388 388
Lines 120013 120064 +51
==========================================
+ Hits 94197 94405 +208
+ Misses 25816 25659 -157
Continue to review full report at Codecov.
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behlendorf
approved these changes
Jun 24, 2019
behlendorf
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Status: Code Review Needed
behlendorf
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Status: Accepted
tonyhutter
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Dec 24, 2019
With the new parallel allocators scheme, there is a possibility for a problem where two threads, allocating from the same allocator at the same time, conflict with each other. There are two primary cases to worry about. First, another thread working on another allocator activates the same metaslab that the first thread was trying to activate. This results in the first thread needing to go back and reselect a new metaslab, even though it may have waited a long time for this metaslab to load. Second, another thread working on the same allocator may have activated a different metaslab while the first thread was waiting for its metaslab to load. Both of these cases can cause the first thread to be significantly delayed in issuing its IOs. The second case can also cause metaslab load/unload churn; because the metaslab is loaded but not fully activated, we never set the selected_txg, which results in the metaslab being immediately unloaded again. This process can repeat many times, wasting disk and cpu resources. This is more likely to happen when the IO of the first thread is a larger one (like a ZIL write) and the other thread is doing a smaller write, because it is more likely to find an acceptable metaslab quickly. There are two primary changes. The first is to always proceed with the allocation when returning from metaslab_activate if we were preempted in either of the ways described in the previous section. The second change is to set the selected_txg before we do the call to activate so that even if the metaslab is not used for an allocation, we won't immediately attempt to unload it. Reviewed by: Jerry Jelinek <[email protected]> Reviewed by: Matt Ahrens <[email protected]> Reviewed by: Serapheim Dimitropoulos <[email protected]> Reviewed by: Brian Behlendorf <[email protected]> Signed-off-by: Paul Dagnelie <[email protected]> External-issue: DLPX-61314 Closes openzfs#8843
tonyhutter
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Dec 27, 2019
With the new parallel allocators scheme, there is a possibility for a problem where two threads, allocating from the same allocator at the same time, conflict with each other. There are two primary cases to worry about. First, another thread working on another allocator activates the same metaslab that the first thread was trying to activate. This results in the first thread needing to go back and reselect a new metaslab, even though it may have waited a long time for this metaslab to load. Second, another thread working on the same allocator may have activated a different metaslab while the first thread was waiting for its metaslab to load. Both of these cases can cause the first thread to be significantly delayed in issuing its IOs. The second case can also cause metaslab load/unload churn; because the metaslab is loaded but not fully activated, we never set the selected_txg, which results in the metaslab being immediately unloaded again. This process can repeat many times, wasting disk and cpu resources. This is more likely to happen when the IO of the first thread is a larger one (like a ZIL write) and the other thread is doing a smaller write, because it is more likely to find an acceptable metaslab quickly. There are two primary changes. The first is to always proceed with the allocation when returning from metaslab_activate if we were preempted in either of the ways described in the previous section. The second change is to set the selected_txg before we do the call to activate so that even if the metaslab is not used for an allocation, we won't immediately attempt to unload it. Reviewed by: Jerry Jelinek <[email protected]> Reviewed by: Matt Ahrens <[email protected]> Reviewed by: Serapheim Dimitropoulos <[email protected]> Reviewed by: Brian Behlendorf <[email protected]> Signed-off-by: Paul Dagnelie <[email protected]> External-issue: DLPX-61314 Closes openzfs#8843
tonyhutter
pushed a commit
that referenced
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Jan 23, 2020
With the new parallel allocators scheme, there is a possibility for a problem where two threads, allocating from the same allocator at the same time, conflict with each other. There are two primary cases to worry about. First, another thread working on another allocator activates the same metaslab that the first thread was trying to activate. This results in the first thread needing to go back and reselect a new metaslab, even though it may have waited a long time for this metaslab to load. Second, another thread working on the same allocator may have activated a different metaslab while the first thread was waiting for its metaslab to load. Both of these cases can cause the first thread to be significantly delayed in issuing its IOs. The second case can also cause metaslab load/unload churn; because the metaslab is loaded but not fully activated, we never set the selected_txg, which results in the metaslab being immediately unloaded again. This process can repeat many times, wasting disk and cpu resources. This is more likely to happen when the IO of the first thread is a larger one (like a ZIL write) and the other thread is doing a smaller write, because it is more likely to find an acceptable metaslab quickly. There are two primary changes. The first is to always proceed with the allocation when returning from metaslab_activate if we were preempted in either of the ways described in the previous section. The second change is to set the selected_txg before we do the call to activate so that even if the metaslab is not used for an allocation, we won't immediately attempt to unload it. Reviewed by: Jerry Jelinek <[email protected]> Reviewed by: Matt Ahrens <[email protected]> Reviewed by: Serapheim Dimitropoulos <[email protected]> Reviewed by: Brian Behlendorf <[email protected]> Signed-off-by: Paul Dagnelie <[email protected]> External-issue: DLPX-61314 Closes #8843
allanjude
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Apr 28, 2020
With the new parallel allocators scheme, there is a possibility for a problem where two threads, allocating from the same allocator at the same time, conflict with each other. There are two primary cases to worry about. First, another thread working on another allocator activates the same metaslab that the first thread was trying to activate. This results in the first thread needing to go back and reselect a new metaslab, even though it may have waited a long time for this metaslab to load. Second, another thread working on the same allocator may have activated a different metaslab while the first thread was waiting for its metaslab to load. Both of these cases can cause the first thread to be significantly delayed in issuing its IOs. The second case can also cause metaslab load/unload churn; because the metaslab is loaded but not fully activated, we never set the selected_txg, which results in the metaslab being immediately unloaded again. This process can repeat many times, wasting disk and cpu resources. This is more likely to happen when the IO of the first thread is a larger one (like a ZIL write) and the other thread is doing a smaller write, because it is more likely to find an acceptable metaslab quickly. There are two primary changes. The first is to always proceed with the allocation when returning from metaslab_activate if we were preempted in either of the ways described in the previous section. The second change is to set the selected_txg before we do the call to activate so that even if the metaslab is not used for an allocation, we won't immediately attempt to unload it. Reviewed by: Jerry Jelinek <[email protected]> Reviewed by: Matt Ahrens <[email protected]> Reviewed by: Serapheim Dimitropoulos <[email protected]> Reviewed by: Brian Behlendorf <[email protected]> Signed-off-by: Paul Dagnelie <[email protected]> External-issue: DLPX-61314 Closes openzfs#8843 Signed-off-by: Bryant G. Ly <[email protected]> Conflicts: module/zfs/metaslab.c
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Motivation and Context
With the new parallel allocators scheme, there is a possibility for a problem where two threads, allocating from the same allocator at the same time, conflict with each other. There are two primary cases to worry about. First, another thread working on another allocator activates the same metaslab that the first thread was trying to activate. This results in the first thread needing to go back and reselect a new metaslab, even though it may have waited a long time for this metaslab to load. Second, another thread working on the same allocator may have activated a different metaslab while the first thread was waiting for its metaslab to load. Both of these cases can cause the first thread to be significantly delayed in issuing its IOs. The second case can also cause metaslab load/unload churn; because the metaslab is loaded but not fully activated, we never set the selected_txg, which results in the metaslab being immediately unloaded again. This process can repeat many times, wasting disk and cpu resources. This is more likely to happen when the IO of the first thread is a larger one (like a ZIL write) and the other thread is doing a smaller write, because it is more likely to find an acceptable metaslab quickly.
Description
There are two primary changes. The first is to always proceed with the allocation when returning from metaslab_activate if we were preempted in either of the ways described in the previous section. The second change is to set the selected_txg before we do the call to activate so that even if the metaslab is not used for an allocation, we won't immediately attempt to unload it.
How Has This Been Tested?
Passes the zfs-test suite and zloop, and has been performance tested extensively on Illumos, where it resolved a number of performance anomalies.
Types of changes
Checklist:
Signed-off-by.