Perf-improvement: CreateEtag method

[ElizabethOkerio]

The CreateEtag method creates an ETag for a given entity. This method is called for every resource and seems to take-up a lot of CPU time even in cases where ETags are not being created.

This is how the CreateEtag method looks like:

 public virtual string CreateETag(ResourceContext resourceContext)
 {
     if (resourceContext == null)
     {
         throw Error.ArgumentNull(nameof(resourceContext));
     }

     if (resourceContext.Request != null)
     {
         IEdmModel model = resourceContext.EdmModel;
         IEdmNavigationSource navigationSource = resourceContext.NavigationSource;

         IEnumerable<IEdmStructuralProperty> concurrencyProperties;
         if (model != null && navigationSource != null)
         {
             concurrencyProperties = model.GetConcurrencyProperties(navigationSource).OrderBy(c => c.Name);
         }
         else
         {
             concurrencyProperties = Enumerable.Empty<IEdmStructuralProperty>();
         }

         IDictionary<string, object> properties = new Dictionary<string, object>();
         foreach (IEdmStructuralProperty etagProperty in concurrencyProperties)
         {
             properties.Add(etagProperty.Name, resourceContext.GetPropertyValue(etagProperty.Name));
         }

         return resourceContext.Request.CreateETag(properties, resourceContext.TimeZone);
     }

     return null;
 }

We call return resourceContext.Request.CreateETag(properties, resourceContext.TimeZone) every time even when the properties are null. This call involves getting the ETag handler from the DI container which seems to take up some CPU time.. Adding a check to only call this method when the concurrency properties are not null seems to improve CPU usage a bit.

Comparing the before and after CPU usage, you can see an improvement in CPU usage of about 1.37%

[gathogojr]

@ElizabethOkerio In your pull request description you state:

Having another cache in Microsoft.AspNetCore.OData to avoid calling this method: model.GetAnnotationValue(model); every time seems to have a slight improvement on CPU usage.

Where is the change for the other cache?

[ElizabethOkerio]

@gathogojr I decided to leave it out since there is on-going work to improve the VersioningDictionary from the Edm lib.

[habbes]

@ElizabethOkerio could you share the CPU usage for the new changes.