On Wed, Mar 9, 2022 at 11:03 PM Peter Xu <peterx@xxxxxxxxxx> wrote: > > On Wed, Mar 09, 2022 at 03:39:44PM -0800, David Matlack wrote: > > On Tue, Mar 8, 2022 at 11:31 PM Peter Xu <peterx@xxxxxxxxxx> wrote: > > > > > > On Mon, Mar 07, 2022 at 03:39:37PM -0800, David Matlack wrote: > > > > On Sun, Mar 6, 2022 at 9:22 PM Peter Xu <peterx@xxxxxxxxxx> wrote: > > > > > > > > > > Hi, David, > > > > > > > > > > Sorry for a very late comment. > > > > > > > > > > On Thu, Feb 03, 2022 at 01:00:28AM +0000, David Matlack wrote: > > > > > > Performance > > > > > > ----------- > > > > > > > > > > > > Eager page splitting moves the cost of splitting huge pages off of the > > > > > > vCPU thread and onto the thread invoking VM-ioctls to configure dirty > > > > > > logging. This is useful because: > > > > > > > > > > > > - Splitting on the vCPU thread interrupts vCPUs execution and is > > > > > > disruptive to customers whereas splitting on VM ioctl threads can > > > > > > run in parallel with vCPU execution. > > > > > > > > > > > > - Splitting on the VM ioctl thread is more efficient because it does > > > > > > no require performing VM-exit handling and page table walks for every > > > > > > 4K page. > > > > > > > > > > > > To measure the performance impact of Eager Page Splitting I ran > > > > > > dirty_log_perf_test with tdp_mmu=N, various virtual CPU counts, 1GiB per > > > > > > vCPU, and backed by 1GiB HugeTLB memory. > > > > > > > > > > > > To measure the imapct of customer performance, we can look at the time > > > > > > it takes all vCPUs to dirty memory after dirty logging has been enabled. > > > > > > Without Eager Page Splitting enabled, such dirtying must take faults to > > > > > > split huge pages and bottleneck on the MMU lock. > > > > > > > > > > > > | "Iteration 1 dirty memory time" | > > > > > > | ------------------------------------------- | > > > > > > vCPU Count | eager_page_split=N | eager_page_split=Y | > > > > > > ------------ | -------------------- | -------------------- | > > > > > > 2 | 0.310786549s | 0.058731929s | > > > > > > 4 | 0.419165587s | 0.059615316s | > > > > > > 8 | 1.061233860s | 0.060945457s | > > > > > > 16 | 2.852955595s | 0.067069980s | > > > > > > 32 | 7.032750509s | 0.078623606s | > > > > > > 64 | 16.501287504s | 0.083914116s | > > > > > > > > > > > > Eager Page Splitting does increase the time it takes to enable dirty > > > > > > logging when not using initially-all-set, since that's when KVM splits > > > > > > huge pages. However, this runs in parallel with vCPU execution and does > > > > > > not bottleneck on the MMU lock. > > > > > > > > > > > > | "Enabling dirty logging time" | > > > > > > | ------------------------------------------- | > > > > > > vCPU Count | eager_page_split=N | eager_page_split=Y | > > > > > > ------------ | -------------------- | -------------------- | > > > > > > 2 | 0.001581619s | 0.025699730s | > > > > > > 4 | 0.003138664s | 0.051510208s | > > > > > > 8 | 0.006247177s | 0.102960379s | > > > > > > 16 | 0.012603892s | 0.206949435s | > > > > > > 32 | 0.026428036s | 0.435855597s | > > > > > > 64 | 0.103826796s | 1.199686530s | > > > > > > > > > > > > Similarly, Eager Page Splitting increases the time it takes to clear the > > > > > > dirty log for when using initially-all-set. The first time userspace > > > > > > clears the dirty log, KVM will split huge pages: > > > > > > > > > > > > | "Iteration 1 clear dirty log time" | > > > > > > | ------------------------------------------- | > > > > > > vCPU Count | eager_page_split=N | eager_page_split=Y | > > > > > > ------------ | -------------------- | -------------------- | > > > > > > 2 | 0.001544730s | 0.055327916s | > > > > > > 4 | 0.003145920s | 0.111887354s | > > > > > > 8 | 0.006306964s | 0.223920530s | > > > > > > 16 | 0.012681628s | 0.447849488s | > > > > > > 32 | 0.026827560s | 0.943874520s | > > > > > > 64 | 0.090461490s | 2.664388025s | > > > > > > > > > > > > Subsequent calls to clear the dirty log incur almost no additional cost > > > > > > since KVM can very quickly determine there are no more huge pages to > > > > > > split via the RMAP. This is unlike the TDP MMU which must re-traverse > > > > > > the entire page table to check for huge pages. > > > > > > > > > > > > | "Iteration 2 clear dirty log time" | > > > > > > | ------------------------------------------- | > > > > > > vCPU Count | eager_page_split=N | eager_page_split=Y | > > > > > > ------------ | -------------------- | -------------------- | > > > > > > 2 | 0.015613726s | 0.015771982s | > > > > > > 4 | 0.031456620s | 0.031911594s | > > > > > > 8 | 0.063341572s | 0.063837403s | > > > > > > 16 | 0.128409332s | 0.127484064s | > > > > > > 32 | 0.255635696s | 0.268837996s | > > > > > > 64 | 0.695572818s | 0.700420727s | > > > > > > > > > > Are all the tests above with ept=Y (except the one below)? > > > > > > > > Yes. > > > > > > > > > > > > > > > > > > > > > Eager Page Splitting also improves the performance for shadow paging > > > > > > configurations, as measured with ept=N. Although the absolute gains are > > > > > > less since ept=N requires taking the MMU lock to track writes to 4KiB > > > > > > pages (i.e. no fast_page_fault() or PML), which dominates the dirty > > > > > > memory time. > > > > > > > > > > > > | "Iteration 1 dirty memory time" | > > > > > > | ------------------------------------------- | > > > > > > vCPU Count | eager_page_split=N | eager_page_split=Y | > > > > > > ------------ | -------------------- | -------------------- | > > > > > > 2 | 0.373022770s | 0.348926043s | > > > > > > 4 | 0.563697483s | 0.453022037s | > > > > > > 8 | 1.588492808s | 1.524962010s | > > > > > > 16 | 3.988934732s | 3.369129917s | > > > > > > 32 | 9.470333115s | 8.292953856s | > > > > > > 64 | 20.086419186s | 18.531840021s | > > > > > > > > > > This one is definitely for ept=N because it's written there. That's ~10% > > > > > performance increase which looks still good, but IMHO that increase is > > > > > "debatable" since a normal guest may not simply write over the whole guest > > > > > mem.. So that 10% increase is based on some assumptions. > > > > > > > > > > What if the guest writes 80% and reads 20%? IIUC the split thread will > > > > > also start to block the readers too for shadow mmu while it was not blocked > > > > > previusly? From that pov, not sure whether the series needs some more > > > > > justification, as the changeset seems still large. > > > > > > > > > > Is there other benefits besides the 10% increase on writes? > > > > > > > > Yes, in fact workloads that perform some reads will benefit _more_ > > > > than workloads that perform only writes. > > > > > > > > The reason is that the current lazy splitting approach unmaps the > > > > entire huge page on write and then maps in the just the faulting 4K > > > > page. That means reads on the unmapped portion of the hugepage will > > > > now take a fault and require the MMU lock. In contrast, Eager Page > > > > Splitting fully splits each huge page so readers should never take > > > > faults. > > > > > > > > For example, here is the data with 20% writes and 80% reads (i.e. pass > > > > `-f 5` to dirty_log_perf_test): > > > > > > > > | "Iteration 1 dirty memory time" | > > > > | ------------------------------------------- | > > > > vCPU Count | eager_page_split=N | eager_page_split=Y | > > > > ------------ | -------------------- | -------------------- | > > > > 2 | 0.403108098s | 0.071808764s | > > > > 4 | 0.562173582s | 0.105272819s | > > > > 8 | 1.382974557s | 0.248713796s | > > > > 16 | 3.608993666s | 0.571990327s | > > > > 32 | 9.100678321s | 1.702453103s | > > > > 64 | 19.784780903s | 3.489443239s | > > > > > > It's very interesting to know these numbers, thanks for sharing that. > > > > > > Above reminded me that eager page split actually does two things: > > > > > > (1) When a page is mapped as huge, we "assume" this whole page will be > > > accessed in the near future, so when split is needed we map all the > > > small ptes, and, > > > > Note, this series does not add this behavior to the fault path. > > > > > > > > (2) We move the split operation from page faults to when enable-dirty-track > > > happens. > > > > > > We could have done (1) already without the whole eager split patchsets: if > > > we see a read-only huge page on a page fault, we could populat the whole > > > range of ptes, only marking current small pte writable but leaving the rest > > > small ptes wr-protected. I had a feeling this will speedup the above 19.78 > > > seconds (64 cores case) fairly much too to some point. > > > > The problem with (1) is that it still requires faults to split the > > huge pages. Those faults will need to contend for the MMU lock, and > > will hold the lock for longer than they do today since they are doing > > extra work. > > Right. But that overhead is very limited, IMHO.. per the numbers, it's the > 20sec and 18sec difference for full write faults. > > The thing is either split or vcpu will take the write lock anyway. So it > either contends during split, or later. Without tdp (so never PML) it'll > need a slow page fault anyway even if split is done before hand.. > > > > > I agree there might be some benefit for workloads, but for write-heavy > > workloads there will still be a "thundering herd" problem when dirty > > logging is first enable. I'll admit though I have not testing this > > approach. > > Indeed that's majorly the core of my question, on why this series cares > more on write than read workloads. To me they are all possible workloads, > but maybe I'm wrong? This series benefits heavy writes, but it may not > benefit (or even make it slower on) heavy reads. It's not that either workload is more important than the other, or that we care about one more than the other. It's about the effects of dirty logging on each workload. Eager page splitting is all about avoiding the large (like 99% degradation), abrupt, scales-with-the-number-of-vcpus, drop in performance when dirty logging is enabled. This drop can be catastrophic to customer workloads, causing application failure. Eager page splitting may introduce higher TLB miss costs for read-heavy workloads, making them worse than without Eager page splitting, but that is not something that causes application failure. Maybe this is bias from working for a cloud provider, but it's much better to have predictable performance for all workloads (even if it's slightly worse for some workloads) than a system that causes catastrophic failure for some workloads. Now that being said, KVM's shadow paging can still cause "catastrophic failure" since it requires the write lock to handle 4KiB write-protection faults. That's something that would be worth addressing as well, but separately. > > The tdp mmu case is more persuasive in that: > > (a) Split runs concurrently on vcpu faults, > > (b) When with PML the tdp mmu case could completely avoid the small write > page faults. > > All these benefits do not exist for shadow mmu. Here's how I reason about the benefits of eager page splitting for the shadow MMU. During dirty logging the shadow MMU suffers from: (1) Write-protection faults on huge pages that take the MMU lock to unmap the huge page, map a 4KiB page, and update the dirty log. (2) Non-present faults caused by (1) that take the MMU lock to map in the missing page. (3) Write-protection faults on 4KiB pages that take the MMU lock to make the page writable and update the dirty log. The benefit of eager page splitting is to eliminate (1) and (2). (BTW, maybe to address (3) we could try to handle these write-protection faults under the MMU read lock.) > > I don't think I'm against this series.. I think at least with the series > we can have matching feature on tdp and !tdp, meanwhile it still benefits a > lot on read+write mix workloads are you proved in the follow up tests (PS: > do you think that should be mentioned in the cover letter too?). Yes, will do! > > IMHO when a performance feature is merged, it'll be harder to be removed > because once merged it'll be harder to be proved wrong. I hope it'll be > worth it when it gets merged and being maintained in upstream kvm, so I > raised these questions, hope that we at least thoroughly discuss the pros > and cons. > > > > > An alternative approach to handling read-heavy workloads we're looking > > at is to perform dirty logging at 2M. > > I agree that's still something worth exploring. > > > > > > > > > Entry (1) makes a lot of sense to me; OTOH I can understand entry (2) but > > > not strongly. > > > > > > My previous concern was majorly about having readers being blocked during > > > splitting of huge pages (not after). For shadow mmu, IIUC the split thread > > > will start to take write lock rather than read lock (comparing to tdp mmu), > > > hence any vcpu page faults (hmm, not only reader but writters too I think > > > with non-present pte..) will be blocked longer than before, am I right? > > > > > > Meanwhile for shadow mmu I think there can be more page tables to walk > > > comparing to the tdp mmu for a single huge page to split? My understanding > > > is tdp mmu pgtables are mostly limited by the number of address spaces (?), > > > but shadow pgtables are per-task. > > > > Or per-L2 VM, in the case of nested virtualization. > > > > > So I'm not sure whether for a guest with > > > a lot of active tasks sharing pages, the split thread can spend quite some > > > time splitting, during which time with write lock held without releasing. > > > > The eager page splitting code does check for contention and drop the > > MMU lock in between every SPTE it tries to split. But there still > > might be some increase in contention due to eager page splitting. > > Ah right.. > > > > > > > > > These are kind of against the purpose of eager split on shadowing, which is > > > to reduce influence for guest vcpu threads? But I can't tell, I could have > > > missed something else. It's just that when applying the idea to shadow mmu > > > it sounds less attractive than the tdp mmu case. > > > > The shadow MMU is also used for Nested Virtualization, which is a bit > > different from "typical" shadow paging (ept/npt=N) because VMs tend > > not to share pages, their page tables are fairly static (compared to > > process page tables), and they tend to be longer lived. So there will > > not be as much steady-state MMU lock contention that would be > > negatively impacted by eager page splitting. > > > > You might be right though that ept/npt=N has enough steady-state MMU > > lock contention that it will notice eager page splitting. But then > > again, it would be even more affected by lazy splitting unless the > > guest is doing very few writes. > > Yes, indeed I see no easy solution to this due to the same lock contention. > > Thanks, > > -- > Peter Xu >
